Vanguard did not offer me the opportunity to designate this as my RMD. SO how does Uncle Sugar know I satisfied the RMD?

I got the same thing on my first RMD when I was with TDA, so I asked TDA. The answer I got back in short order (TDA's customer service was excellent, both in timeliness and accuracy and cogency) was that they can't really say whether my withdrawal satisfied my RMD requirement because they don't know what other IRAs I might have with other brokers, and those IRAs contribute to my total RMD for any year. The withdrawal would be reflected on the 1099 TDA sends out for the tax year, and that 1099-reported withdrawal can be used to satisfy the IRS--which knows what my RMD is because they know what I have in all my TIRA accounts.

Sure enough, that happened with my 1099 for that year (and subsequent years), and the IRS has been satisfied each year.

TDA also had no problem tracking my IRA withdrawals from them and telling me how much RMD I had left to withdraw from my account with them (tacitly assuming that account was my only account). So far, it's that way with Schwab, too.

I suspect your situation with V'guard and mine with erstwhile TDA are typical IRA-wide.

Eric Hines