Two things about the vanguard solo 401k:
1) They'll waive the fee once you reach some status (flagship maybe) which happens when your whole household has 50k with them.
2) You can only invest in investor class shares, not the lower expense ration admiral class. Doesn't matter if you're doing a target date, but it would if you want to do total stock, total bond, or the international equivalents. I'm leaning towards going with fidelity, which has the spartan funds that are similar to vanguard and do allow you to use advantage class (like admiral class) in the solo 401k.