The answer is "YES" you will lose all vacuum driven instruments. Just look at the Vacuum gauge on idle or even slightly higher power settings!!
Having had a Vacuum Pump failure, in IMC, on an ILS it isnt funny. In fact, had we not been near cloud base, I dread to think of the potential outcome. For what its worth I have an ATPL!
I will now never, normally, fly a single in IMC because, when you have your engine failure, you will lose your Attitude Indicator your Directional Indicator and possibly your de icing equipment. It is rare for singles to have 2 Vacuum Systems.
If you survive all this you will then be faced with a forced landing with, possibly, an exteremly low cloud base (otherwise why not stay below?).
Why do you think God made aeroplanes with 2 engines, 2 altimeters,2 Vacuum systems, 2 Generators, 2 VORs, 2 Radios and even 2 pilots?
I really do think that people who fly singles in serious IMC are stretching their luck sometimes.