[doviatt]

I wouldn't say you are doing anything wrong. Just learning what works and what doesn't.
I haven't transferred my vacuum gauge into my current vehicle but when I used it (more than the MPGuino) I always tried to get as much vacuum without loosing speed during the steady portions of the trip. I would get to highway speed and then back off the pedal, just a bit, to increase vacuum and reduce fuel. This was a dynamic process and always required me adjusting to the terrain. It is a subtle technique and takes practice to know when you have backed off too much and deceleration starts.

Edit: I never considered target to mean speed or throttle position. Just target minimal fuel usage.