Does the adjustable Vac. Advance adjust the amount of timing brought in - or the level of vacuum needed to activate it?

Yes.

depends on the brand you get. the main purpose is to be able to adjust at what vacuum level the advance occurs. some units also come with a stair stepped cam unit that pushes against the rod so you can adjust how much advance it will give.

Everyone is correct..... :D
Probably the most popular (Crane) comes with a mechanical limiter for travel and an adjustable spring rate for the vacuum to 'overcome'. The combination limits total advance (mechanical) and the point where it starts to move (higher/lower spring rate). The mechanical dodad moves the "rod" into the can and thus shortens the total travel. The spring rate on the diaphragm is adjusted by a set screw accessible through the vacuum connection with an allen wrench.

Thanks, it is a Crane so I can play now.