It seems to me that as a singing bassist, I'm fairly screwed. I get the impression that vented moulded plugs are pretty much the only way? Other than ear defenders which, errm, no.
Up to now I've been reliant on, and happy with, my trusty old Etymotic ER20 plugs. They certainly do the trick attenuation-wise, but on the basis of what I've read above I'm thinking maybe they're not such a good idea for singing gigs..?