Filed Pursuant to Rule 424(b)(3)
Registration No. 333-215288

PROSPECTUS SUPPLEMENT NO. 7

(to Prospectus dated May 9, 2017)

Vistra Energy Corp.

168,779,076 Shares of Common Stock

This prospectus supplement supplements the prospectus dated May 9, 2017 (as supplemented to date, the Prospectus), which forms a part of our Registration Statement on Form S-1 (Registration No. 333-215288). This prospectus supplement is being filed to update and supplement the information in the Prospectus with information contained in our Current Report on Form 8-K, filed with the Securities and Exchange Commission on October 13, 2017 (the Current Report). Accordingly, we have attached the Current Report to this prospectus supplement.

The Prospectus and this prospectus supplement relate solely to 168,779,076 shares of Vistra Energy Corp. common stock, par value $.01 per share, which we refer to as our common stock or the Vistra Energy common stock, which may be offered for resale from time to time by the stockholders named under the heading “Principal and Selling Stockholders” in the Prospectus, whom we refer to as the selling stockholders. The shares of our common stock offered under the Prospectus, as supplemented by this prospectus supplement, may be resold by the selling stockholders at fixed prices, prevailing market prices at the times of sale, prices related to such prevailing market prices, varying prices determined at the times of sale or negotiated prices and, accordingly, we cannot determine the price or prices at which shares of our common stock may be resold. The shares of our common stock offered by the Prospectus and this prospectus supplement may be resold by the selling stockholders directly to investors or to or through underwriters, dealers or other agents, as described in more detail in the Prospectus. For more information, see the section entitled “Plan of Distribution” in the Prospectus. We do not know if, when or in what amounts a selling stockholder may offer shares of our common stock for resale. The selling stockholders may resell all, some or none of the shares of our common stock offered by the Prospectus, as supplemented by this prospectus supplement, in one or multiple transactions.

This prospectus supplement should be read in conjunction with the Prospectus. This prospectus supplement updates and supplements the information in the Prospectus. If there is any inconsistency between the information in the Prospectus and this prospectus supplement, you should rely on the information in this prospectus supplement.

Our common stock is listed on the New York Stock Exchange, or NYSE, under the symbol “VST.” On October 12, 2017, the closing sales price of our common stock as reported on the NYSE was $19.08 per share.

Investing in our common stock involves risks. Before making a decision to invest in our common stock, you should carefully consider the information referred to under the heading “Risk Factors” beginning on page 21 of the Prospectus.

Neither the Securities and Exchange Commission nor any state or other securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is October 13, 2017.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): October 1, 2017

VISTRA ENERGY CORP.

(Exact name of registrant as specified in its charter)

(214) 812-4600

(Registrant’s telephone number, including area code)

N/A

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company  ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.  ☐

Termination and Settlement of Alcoa Contracts

On October 10, 2017, subsidiaries of Vistra Energy Corp. (collectively the “Vistra Parties”) entered into a Separation and Settlement Agreement (the “Settlement Agreement”) with Alcoa Corporation and Alcoa USA Corp. (collectively, the “Alcoa Parties”). Pursuant to the Settlement Agreement, the Vistra Parties and the Alcoa Parties agreed to early termination of a series of agreements related to industrial operations near Rockdale, Texas, thereby ending their contractual relationship with respect to the power generation plant known as Sandow Unit 4 and the mine known as Three Oaks Mine. The terminated agreements were scheduled to terminate in 2038 absent the Settlement Agreement. Among other things, the Alcoa Parties made a cash payment to the Vistra Parties in the amount of $237.5 million and transferred certain real property and related assets to the Vistra Parties, the Vistra Parties agreed to assume and be responsible for certain liabilities and asset retirement obligations related to Sandow Unit 4 (including certain related common facilities), the Three Oaks Mine and other property transferred from Alcoa and both parties released one another from any obligations and claims under the terminated agreements. The transactions under the Settlement Agreement are effective as of October 1, 2017.

We expect to record the impacts of the Settlement Agreement in our financial results for the fourth quarter of 2017, which would include the receipt of the cash payment, the acquisition of real property and the incurrence of certain liabilities and asset retirement obligations, along with the elimination of a related electric supply contract intangible asset on our consolidated balance sheet. We currently estimate that the aggregate impacts related to the Settlement Agreement will result in a gain in the period.

Retirement of Certain Generation Units

In October 2017, Vistra Energy Corp.’s power generation business, Luminant, announced plans to retire its Monticello, Sandow 4, Sandow 5 and Big Brown generation units. Luminant decided to retire these units given they are projected to be uneconomic based on current market conditions and given the significant environmental costs associated with operating such units. In the case of the Sandow units, the decision also reflected the execution of the Settlement Agreement. As part of the retirement process, Luminant has filed (or will file) notices with the Electric Reliability Council of Texas (“ERCOT”), which will trigger a reliability review regarding such proposed retirements. If, at the end of the applicable ERCOT reliability review period, ERCOT determines the units are not needed for reliability, Luminant would expect to cease plant operations at Sandow and Monticello in January 2018 and at Big Brown in February 2018, which would result in approximately 4,200 megawatts being taken offline by February 2018.

With respect to the Monticello plant, we expect to record one-time charges of approximately $25 million in the third quarter of 2017 related to the expected retirement, including employee-related severance costs and non-cash charges for materials inventory and the acceleration of certain of Luminant’s asset retirement obligations.

With respect to the Sandow 4, Sandow 5 and Big Brown plants, we expect to record one-time charges of approximately $70 to 90 million in the fourth quarter of 2017 related to the expected retirements, including employee-related severance costs, non-cash charges for writing off materials inventory and a contract intangible asset associated with Big Brown. We expect to record additional one-time charges in the fourth quarter of 2017 related to changes in the timing and amounts of asset retirement obligations for mining and plant-related reclamation at these facilities.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.