alphaalpha
A lot of the time, an AP is no good at all with a vac failure. I have a vac primary horizon and an electric TC and DI but the AP uses both the vac horizon and the electric DI!
Also, when tracking a VOR one still has to fly a heading primarily so I don't think any AP will be able to track a VOR if the DI has gone...
Re GPS as a backup... you get a LOT more out of a GPS if you use it as a primary instrument, concurrently with the others, than if you start using it only when everything has turned to ****.