Leave everything as-is when you're setting timing with vacuum. Since you're setting max advance at speed, you want the system to work just like it does on the road.
I have my DGV line running to the dizzy, but the diaphragm is kaput...so no air leaks, but it's not working either. One of these days I'll fix it, but for now I'm running without vacuum advance, and living with the power and economy penalty at the lower end. The neat thing about the vacuum timing method is you can find your best advance throughout the whole rpm range, and tweak your curve with heavier or lighter springs. Gotta be patient to do that, though, so I just set the max advance and let the curve fall where it may. ('Course, that's probably why I'm only getting 21 mpg, too.)
I have a little nipple threaded into the intake manifold just for the vacuum gauge hookup.