One of the things that I have found when teaching the IMC rating is that some people can fly the aircraft better on partal panel than full panel in the early stages of there IF training.
I can only think that they have less think about with the AI and DI failed !.
The real danger of vac system failure is in the fast identification of the failure as the gyros lose ridgidity this requires a very good panel scan to pick up before you have to try the recovery from unusual attitudes.