The vac lock system on MB's is actually a pretty stout system when you consider how old all the rubber is. Sounds like you might have a leak somewhere if everything takes that long to unlock?
Anyway, what I did when I installed a cheapo alarm with remote door locks is just mount ONE solenoid (bought from mandsenterprise on ebay for $3.99 each, bought a few of them, HIGHLY recommend these sellers) in the drivers door and kept the vac system intact.
This gives me power door locks that remote lock/unlock everything (the solenoid operates the unlock on the drivers door and that vacuum unlocks the rest of the doors and vice-versa).
Here's the solenoid mounted in the lower portion of the drivers door:
And here's how I mounted to the upper lock knob rod (took some creative re-forming of the rod to miss everything in the door):
Since I had my interior apart just like you, running the wiring was cake, but I wouldn't want to do it on all four doors!
Good luck, email if you have any questions I can help with in regards to the above.