Quoted:
how hard would it be to install a electro magnet to the four corners of the door?
have a battery backup inside the vault, in case they unplug the power leads to the thing...
those electro magnets can be very very VERY strong.
Not a bad idea. I know the type you're talking about, on commercial doors with key-card/RFID fob access etc.
The main problem I see is that they require several square inches of surface area to get the needed pull, which means designing the safe from scratch to use them. I don't think it's something you could retrofit to around the edge of the door on an existing safe.
Honestly, the easiest thing to do are twofold.
1. Bolt the safe down.
2. Position the safe in tight confines so a pry-bar can't be pushed more than a few inches. If you've got only a corner in an open room to work with, make sure the edge of the door that opens, opposite the hinges is against a wall.
Thieves by definition are lazy, otherwise they wouldn't be thieves.
Then all you're up against is a cutting/saw, chisel, and wedge/sledge/ax type attack, on the one exposed side, and the number of those seems to be way lower than the prybar ones on average. IMO, exposed hinges, when backed by a full length flange, or pins aren't bad either, because they are more likely to waste time cutting/sledging them off, only to find it makes no difference. Studs, drywall etc. is just meh... I've got no problem with insurance paying to fix that. It's the guns I'm protecting and I'm emotionally invested in.
My ideal setup would be a concealed closet for the real safe, with a cheap Homak/Stack-On cabinet filled with Century Arms U-Fix-em craptastic milsurp Mosin Nagant's, and maybe one or two cheap-ass Charles Daly-type Walmart imports, an air-rifle, and some cheap electronics.. as the DOOR to the hidden closet.