[niky]

When the ignition cuts out, you have enough vacuum in the brake booster for two or three stops. And if you're moving, you still have steering. Heavy steering, but it's still there.

Electric steering and brake boosters will give you steering and brake assist even if the engine goes off. But if the electronics turn off, as they did with the GM ignition switch issue, then the assists go off, too.

If it's simply an engine problem, as when the engine conks out while driving but you keep it in gear, current systems are adequate.

If you want ultimate redundancy, then you want an extra vacuum pump and reservoir running off a separate motor or being generated through some sort of regenerative braking system.