At least via the web interface for Vanguard the way I do it is start with a buy order, and then when it says "where funded" hit "new deposit" or whatever it is.
They then let me buy whilst the money is inflight.
They then let me buy whilst the money is inflight.
The new app seems to only offer sending money to money market settlement, before waiting two days to transact.
A real pain of a change, unless I'm missing a way to do it.