It actually happened today!! Vacuum failure in IMC.

I'm on a road trip and not near the plane right now, so can't tell you
the name. However, it is an electric pump, not the Precise Flight
model that draws off the manifold (or whatever), and is not dependent
on the throttle. They are still looking at things, and believe that a
shuttle valve either stuck or that something may have somehow gotten
sucked into the plumbing. The backup pump is, itself, working. I 'll
let this answer some of the other queries about what happened, until I
know more.