It actually happened today!! Vacuum failure in IMC.

I don't remember if the a/p was on but I think it was. I know that may
seem weird, but the whole thing was so unexpected (naturally), that I
just focused on acting and my memory is . If it was on, the a/p would
have been on nav mode rather than hdg mode since I had been given a
clearance to a VOR, "then as filed", so I would have dialed in the VOR
freq and set the a/p to it.
Eventually, after getting some no-gyro vectors using the t/c and timed
turns on the compass, and alt instructions, I set up the GPS to CLT and
set the a/p to it. At that point I began to breathe easier.