Set the allocation for your new 401k to a cap-weighted blend of FXAIX, FSTPX, and FSSSX (large/mid/small cap indices). Those three funds are excellent on expenses! In fact, you'd actually do better rolling over your Vanguard 401k to Fidelity than you would rolling it over to a Vanguard IRA because of that (they're even cheaper than Vanguard Admiral funds).
Of course, that's if you're okay with with a 100% domestic equity portfolio -- which, for a mustachian in his 20s, is a perfectly good choice. But if you want international stocks (or bonds, though I wouldn't bother), that's different because your choices suck for those asset classes. In that case, you should still do the allocation I wrote above for your new 401k. But instead of rolling your existing 401k into it, you should keep it (or roll it into a Vanguard IRA, depending on expenses of course) and buy all your international and bonds there.
For example, say you want 70% domestic stock, 20% international stock and 10% bonds, and it's a couple years from now so you have 100K in your Vanguard IRA and 200K in your new Fidelity 401k. In that case, your allocation would be $30K VBTLX, $60K VTIAX and $10K VTSAX in your Vanguard IRA and 100% [cap weighted mix of Fidelity funds above] in your 401k.
(Eventually your 401k might get so much larger than your soon-to-be-IRA that the non-domestic-equity allocation might no longer fit completely in the latter, but you can cross that bridge when you come to it.)