Are we over thinking this...?
Probably ;) Presumably, the amount of money you have is very small relative to what it will be in the future, so minor differences in allocation are not a very big deal right now (I know they seem like big deals though, they did to me too when I first started and had all of like $5k.)
With the funds you mentioned, you can do 50% S&P500 in the 401k; and 25% bonds, 25% international in the IRA. 25% is a nice percentage for bonds and at this point is just as good or even better than 35%.
You have VBTLX (total bond) in the 401k as well, so you can can fine tune your bond allocation there if you want like 401K: 35% ($4,200) in VBTLX, 15% ($1,800) VFIAX | IRA: 25% ($3k) VTSAX, 25% VTIAX ($3k)
Either of those two is good. There are infinite possibilities, but once you have the stock/bond split decided they mostly don't foreseeably make a difference.