Vanguard, at which we have an account, says there is no fee to purchase 6 month T-bills. It seems simpler to just make the purchases through Vanguard, than to open another account at Treasury Direct. Is there a reason to use TD instead of Vanguard or Fidelity?

Assuming you qualify for free auction purchases of Treasuries (T-bills, Notes, TIPS) through Vanguard Brokerage Service, the only reason I can think of for using T-Direct is convenience/minimizing time lag, and I may be out of date on "just-in-time" transfers of cash to Vanguard.

In my experience, it has always been easy to drip money from my bank to Vanguard for fund purchases on a given date. It wasn't possible, as of a couple of years ago, to get an exact amount taken from the bank account for brokerage purchases, so it meant keeping money in a Vanguard Money Market (which paid less than my bank Money Market). Not a big deal, and I haven't checked lately whether you can now set up instant transfers without the expense of wiring money.

One definite advantage of Vanguard would be you can get paper account statements. I must say, I'm somewhat nervous about relying on print-outs for T-Direct account (especially since I'm now trying to use a perfectly good printer that is marginally compatible with my new computer).