It actually happened today!! Vacuum failure in IMC.

Dave,
I'll use this to answer both you and Barry, since you have both raisede
the WASS question, but you have a second one.

The rnavs 5,11, 23, and 29 at CAE have an entry that is "LNAV/VNAV".
I'm new to the panel GPS, having just gotten it about a month before
this trip so, although I have flown the approaches when familiarizing
myself with it, I had not thoroughly studied the GPS approach plates.
Before I left AVL, I'd looked at the plates and saw those notations,
and didn't look further. They require WAAS. I now (for the first
time) see that there are also simply LNAV approaches. I'm more
familiar with ILS than GPS approaches (obviously), and planned on an
ILS approach anyway, so hadn't really studied the GPS approach plates.
As for flying the GPS rather than the ILS into CLT, I've been so
indoctrinated into flying headings rather than "chasing the needle" on
an ILS that I had to rule that out. Although I was wrong about the
need for WAAS, I believed that I had no choice at that point-I'd do the
best I could with the GPS approach. The ceilings and vis were better
at CLT than at CAE (which was close to minimums) so it was less of a
risk (in my mind) to do the GPS approach at CLT even though I wasn't
(erroneously) properly equipped with WAAS.