as far as vibration, I don't think it will buy you anything. It's a full foamed spa with a circ pump on concrete. The only sound you're gonna hear is when the jets are on. Might be differtn if it was on a deck, or floor, or roof.
As far as insulation and moisture, I think you'll be all set. they are designed to be put on a slab. If the tub is dry and you wanted to staple some thick mil platic down as a vapor barrier, I don't see it being a problem. I don't think it's necessary, but couldn't hurt. no?
The portable pad, is a systhentic block or pad that some dealers sell that can be used to place a tub on. Folks who don't want to pour a pad use them. They are relativly light weight and can be moved. I belive Artic makes one, and I saw one at the D-1 Dealer the other day.