The authorization of an additional $1.5 billion for share buybacks by Vistra's board is a strong signal of the company's financial stability and its commitment to returning value to shareholders. This move is likely to be viewed positively by the market, as it not only reflects confidence in the company's future prospects but also directly benefits shareholders by potentially increasing earnings per share and the value of remaining shares. Historically, announcements of share buyback programs or expansions thereof tend to lead to positive short-term movements in a company's stock price, as they are often interpreted as signals of undervaluation or strong future performance.