Assuming that Roth is the right option for your tax situation, you could is invest in VTSAX in a taxable account for now and then shift that into a Roth on Jan 1.
I agree. There's a chance value goes down and you harvest losses, so be sure to not buy VTSAX if this is the case in the ROTH buy something similar (VOO/VFIAX) to avoid a wash sale. If you have gains and are forced to pay a few couple dollars in taxes to cash out and fund ROTH in the new year, welcome to rich people problems 😎
I will say that while I personally like to max a ROTH account in the first few months of a year, there's not a big deal at all in funding it a few hundred bucks a month throughout the year as long as you max it out given the investment growth timeline (decades) and typical index funds. So if you funded taxable and left it as is, and then funded ROTH throughout the year next year that's totally fine.