the problem with running manifold vacuum to the advance is that if you have a hefty cam, there is usually a large difference in vacuum between nuetral and drive. and that changes the advance in the distributor which will change the idle A LOT when you drop it in gear.
With a stockish cam, you have 14 or more inches of vacuum in neutral which will bump the vacuum advance up all the way. when you you drop it in gear, you may drop manifold vacuum to 12 or 13 or more so you are still maxed out on the vacuum advance. so no change in idle.
but put a BIG cam in there where you have 8-11 inches in neutral but only 6 or so inches in gear, the low vacuum, most likely pulsing up and down 2 inches, is going to make you idle timing bounce all over the place and cause all sorts of idle problem.
so yes, there IS a problem with using manifold vacuum depending on what you are running