Thanks for the continued clarification and resources. So it looks like the correct choices are a taxable investment in VTSAX, or opening a new Roth with Vanguard and investing in VTSAX that way. Any thoughts on which is more advantageous, since it sounds to me like they're going to be functionally the same in my situation? I don't plan to touch this money prior to retirement and so don't really care about the flexibility.
If it were me, I would skip the complication of the Roth and keep open the option of being flexible. Along the path of life, you'll maybe have opportunities that a chunk of liquid, easily accessible assets make possible.
Two concrete examples of this...
1. If the stock market corrects big time next year and your windfall loses, say, 30% of its value, you can sell the VTSAX to harvest a capital loss, reinvest the proceeds in some other low cost high quality equity mutual fund, and then put a $3000 capital loss deduction on your tax return every year until you burn off the capital loss. (E.g., your windfall equals $30,000 and you tax loss harvest a $9K capital loss... in this case, you'll put a $3K capital loss on next three years of tax returns.)
2. I hesitate to open this can of worms, but while index funds and tax deferred retirement accounts are really the "best practices" way to march toward financial independence, IRS wealth statistics show (suggest?) that people also achieve financial independence by having the resources available to make very opportunistic investments. BTW, I am not saying you should use your windfall money as the down payment on a Jimmy Johns franchise... but I think it's possible if you keep your eyes open that in 8 years and then again in 26 years, you'll get opportunities to earn really good returns if you have cash available.