Ok... I don't claim to know most of this stuff, but this is my understanding....
Forced induction cars - and all cars IIRC, have a vacuum canister with a one-way check valve that will retain some vacuum for things like the heater controls, emissions control motors, etc. Also realize that a majority of the time, the car is being operated in vacuum (especially when slowing down, thus providing the brake booster with the necessary vacuum). I'm guessing here, but it sounds feasible. In any normal car, at WOT you also have no useable vacuum, and the heater controls still function and such.
Nitrous wet and dry shots.... dry shots spray only the nitrous and involve a modified fuel injection system to compensate. This can be anything from a chip telling your computer to add more fuel when the nitrous is applied (in which case you'll probably have to upgrade fuel system components) to putting the nitrous injector before the MAF sensor, telling the computer there is more oxygen entering and providing more fuel. Wet systems provide both fuel and nitrous at the same time. They may also require an upgraded fuel system. This allows the computer to function normally and let the electronics that come with the nitrous kit do the fuel monitoring.
Nitrous should also be used with a few safety precautions... a switch to monitor fuel pressure is a definate thing to have, unless you want a freak accident to cost you dearly. Wide open throttle switches, RPM window activation switches, etc, control when the nitrous kicks in.
On just about any engine, it's recommended to not kick the nitrous in too early or you'll most likely hurt something. 3000 RPM window switches seem to be the number flying around... therefore when the RPMs reach 3000, it'll kick in the sauce and away you go. Just make sure you watch the tach though... if you think redline comes fast enough now, it'll be worse. I guess with an AOD you could perhaps let it shift itself, but you might want to have that rebuilt as well for a more performance-oriented application such as the one you're proposing.
I don't know the limits on blocks. If you really want to squeeze the hell out of it, spend the money and build a totally new engine from an A4 block with forged internals... you should be good to the upper hundreds of horsepower. If you're just looking for a toy, have the block checked for cracks and stressed areas, rebuild it with good forged internals, and squeeze it.. you *should* be able to safely achieve 450-500hp... I think beyond that the block wants to give up... but I can't remember exactly.
Hope that was some food for thought. You might also want to consider how you're gonna get the power to the pavement....
JS