Hey, I never said it wasn't convoluted. This is, after all, the FAA we're talking about.
Also, your vacuum driven AI meets the TSO for a backup AI. Thus, you have the required backup to the Aspen, and your vacuum AI is the backup (not the primary).
The worst part about all this is that it's OK for a G5 to be your ONLY attitude indicator, but it can't be your backup!
I think it does, but I'm not sure it meets the requirements for a backup. IIRC most of the glass displays can go for 1/2 hour with their own batteries, but I think the backup requires 2 hours. You can buy an extra battery for the G500 TXi to meet the requirement, for example, but the "included" battery doesn't.