Exhibit 8.1

[Letterhead of Simpson Thacher & Bartlett LLP]

January 23, 2018

Vistra Energy Corp.

6555 Sierra Drive

Irving, Texas 75039

Re: Vistra Energy/ Dynegy Merger

Ladies and Gentlemen:

We have acted as counsel to Vistra Energy Corp., a Delaware corporation (“Vistra Energy”), in connection with the Merger, as defined and described in the Agreement and Plan of Merger, dated as of October 29, 2017 (the “Agreement”), by and between Vistra Energy and Dynegy Inc., a Delaware corporation (“Dynegy”). For purposes of this opinion, capitalized terms used and not otherwise defined herein shall have the meaning ascribed thereto in the Agreement. This opinion is being delivered in connection with the filing of the registration statement on Form S-4 (Registration No. 333-222049) (as amended, the “Registration Statement”) filed by Vistra Energy, including the joint proxy statement/information statement and prospectus forming a part thereof, with the Securities and Exchange Commission under the Securities Act of 1933, as amended, relating to the proposed transactions pursuant to the Agreement and to which this opinion appears as an exhibit.

We have examined (i) the Agreement, (ii) the Registration Statement and (iii) the representation letters of Vistra Energy and Dynegy delivered to us in connection with this opinion (the “Representation Letters”). In addition, we have examined, and relied as to matters of fact upon, originals or copies, certified or otherwise identified to our satisfaction, of such records, agreements, documents and other instruments and made such other inquiries as we have deemed necessary or appropriate to enable us to render the opinion set forth below. In such examination, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies, and the authenticity of the originals of such latter documents. We have not, however, undertaken any independent investigation of any factual matter set forth in any of the foregoing.

In rendering such opinion, we have assumed, with your permission, that (i) the Merger will be effected in accordance with the Agreement, (ii) the statements concerning the Merger set forth in the Agreement and the Registration Statement are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time, (iii) the representations made by Vistra Energy and Dynegy in their respective Representation Letters are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time and (iv) any representations made in the Agreement or the Representation Letters “to the knowledge of”, or based on the belief of Vistra Energy and Dynegy or similarly qualified are true, complete and

January 23, 2018 Vistra Energy Corp.

correct and will remain true, complete and correct at all times up to and including the Effective Time, in each case without such qualification. We have also assumed that the parties have complied with and, if applicable, will continue to comply with, the covenants contained in the Agreement.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein and in the Registration Statement, we are of the opinion that the statements made in the Registration Statement under the caption “Material U.S. Federal Income Tax Consequences,” insofar as they purport to constitute summaries of certain provisions of U.S. federal income tax law and regulations or legal conclusions with respect thereto, constitute accurate summaries of such matters described therein in all material respects.

We express our opinion herein only as to those matters specifically set forth above and no opinion should be inferred as to the tax consequences of the Merger under any state, local or foreign law, or with respect to other areas of U.S. federal taxation. We do not express any opinion herein concerning any law other than the federal income tax law of the United States.

We hereby consent to the filing of this opinion as Exhibit 8.1 to the Registration Statement, and to the references to our firm name therein.

Very truly yours,

/s/ Simpson Thacher & Bartlett LLP

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