VTSAX is all the US stocks, weighted by market cap. VFIAX is only the 500 largest ones, but by value that's still around 80% of VTSAX. If you look at a
comparison graph of VTSAX vs. VFIAX you'll see that the two funds move mostly in lockstep. The mid-cap fund is medium-sized companies, which make up about 10% of VTSAX. To do a good replica of VTSAX you'd put 80% in the 500 fund, 10% in the mid-cap fund, and 10% in a small-cap fund. Buying 80/20 large-cap/mid-cap might be a bit closer to VTSAX than just buying 100% large-cap, but still not quite there because you're skipping the small companies and buying the medium-sized ones with double weighting.
You could find a small-cap fund elsewhere (in your IRA, perhaps) to make up for the fact that your 401(k) doesn't have one from Vanguard.
Alternatively there's the
Vanguard Extended Market Index that is designed to be VTSAX minus the top 500 companies. If, for example, you have an IRA that's at least a quarter of the balance of your 401(k) you could do 100% VFIAX in the 401(k) and then buy the Extended Market Index in an amount roughly a quarter of the 401(k) balance to get a more exact replica of VTSAX.
Or you could realize that VTSAX is pretty close in historical performance to VFIAX and just buy only that for your stock allocation.
