If you're a straight 1099 / sole proprietor as an individual you can set up a SEP IRA. If you establish an LLC you can create a simple-IRA. Both of these allow you contribute money tax deferred and are much easier to manage than a 401k. Plus, you can trade them like any other brokerage account.
For a solo 401(k), you probably don't need the administrative stuff that they are providing on top of Vanguard, so you could cut out the middle man and go straight to Vanguard. Of course, you do have to be self-employed to qualify legally to have a solo 401(k).