I agree with alphaalpha about using a GPS to help - the garmin 196 in particular has that VFR instrument page which must be of some help on the "DI" and "Altimeter" they have there, though not sure using GS is a good idea. GPS gets a lot of stick - ie should only be used as a backup etc etc, which is true, but I think it could certainly help once a vaccum pump failiure has already been identified