I have an AudioVox cruise control installed. It works great (I made a reservoir and put it in).

Fact - after not using it or not riding for a couple of days to a couple of weeks, the cruise takes a long time to engage. I have tried engaging it multiple times and just keep pushing the engage button. It will eventually engage.

Question 1. What is the cause? Does the vacuum leak and it needs to build up?

Question 2. Is it better to push the button over and over or to hold it down until it engages. Does anyone really know which is better and why.

 Try a vacuum check valve in line with the vacuum supply from the engine. This will hold vacuum in the reservoir, unless you have another leak. Could be a cheap fix.

Mine sometimes acts similar to the way you're describing.  I find that I have to hold the set button for a second or two rather than just push it.  I think it is just the cruise control taking up all the slack in the cable.  There is quite a bit of movement available to the cable, and the CC doesn't make really quick adjustments, as these would make the normal actuation of it quite jerky.

If I just push the button and let go of the throttle, I find that I'll sometimes drop 10mph or more before the cruise starts accelerating.  But a one or two second push seems to do it.  Then a tap on accel or decel will gain or drop 1mph or so.