Good news, I am sure someone else has probably already mentioned it here, but I figure I'll share.
I just setup a Vanguard Solo Roth 401K today and they now allow you to choose Admiral Shares class funds such as VTSAX.
I chose VTSAX alone as the sole fund, to keep the annual fees minimal at $20 per year -- $20 fee waved after you accumulate $50k in total Vanguard assets between all types of Vanguard accounts.
This is great because they only allowed Investor Shares class funds before and were much higher expense ratios. Love that VTSAX is only .04% and that I now have access to it in Solo 401k!
I decided to go this route instead of eTrade and TD Ameritrade, because I know I'd be tempted to invest in individual stocks on the other platforms and I don't want to do that. I just want to make monthly contributions to VTSAX and be done with it.
Since I also have Roth IRA with Vanguard, I can diversify there if I want to. 55% of my monthly retirement savings will go into the Solo Roth 401K and 45% into the Roth IRA. So this leaves me plenty of room if I want to move some VTI into BND in my IRA account. I would never want more than 45% bond asset allocation anyways.