I'm surprized someone hasn't mentioned freeze to deal with plugin load in such a situation.
As I feel satisfied with each track, I freeze it as I can always unfreeze if i want to change it.
By the time I'm tracking vocals, I'm essentially streaming the tracks with just a few plugins on busses...
I do monitor my inputs from the interface so it is essentially the same as tracking to tape. In earlier times I sent a mix from my board with the vocal input and tape/pc playback both occuring there.