Backup gyros - which do you trust?

Newer vacuum AIs have warning flags, dual vacuum pumps, and vacuum warning
lights on the annunciator panel. If I had an electric AI and a vacuum AI and
they disagreed markedly, I would be suspicious of the one having warning
lights and flags all over it.

Otherwise, comparing them to the other instruments, as you suggested, is the
best approach. AOPA Safety Foundation just ran some tests on how long it
takes a pilot to recognize that his vacuum system has failed, and how long
it takes pilots to recover from those failures. Many times it took over 90
seconds, which is way too long. Some never did recognize the failure.
However, none of them were convinced that the vacuum instruments were
working and that it was the other instruments that had failed. IIRC somewhat
less than half discovered the problem quickly and took quick action. Only
about 1/4 of them covered the failed instrument.