There are cruise control units that do not use any vacume at all. If you are going to use vacume with a cruise control it looks like you need to incorporate a vacume tank, you just can't seem to get around it. I worked on my friends Mercedes and it had a rather large vacume tank behind the back seat that worked the power locks, it seems like you can cobble something together from a junk yard. One system that I have been looking at is from audiovox that uses a completely electronic actuator to manipulate the throttle this can be found at:

http://www.asiapacific.com.my/audiovox/product_car2.html

I have not actually tried to procure such a product, but it looks like a vacumeless setup is available.