Theoretically you could, but the two things serve different purposes, though they sometimes share them a little.
A vang by definition is rigged on the same pivot point as the boom/gooseneck, so as to keep a steady tension on the leech no matter whether you trim or ease the mainsheet.
A preventer, rigged to the rail, is there primarily to prevent a jibe if you bear off too far past the lee. When trimmed in close, it also tensions the leech. But to keep the same leech tension, you'd have to readjust it every time you ease or trim the mainsheet.
So if you want consistent leech tension (and boom height) and also something to help prevent an accidental jibe, then you need two separate block and tackle systems. Also, you may want the vang tension during a jibe, which is something you can't do with just a preventer, which has to be let go and shifted to the new lee side during and after the jibe.
Even with mid-boom sheeting (and traveler?), which sort of acts like a vang farther out than boom-end sheeting does, you'd still need a vang *and* a preventer if you really want the advantages of both.
Last edited by nolatom; 02-04-2008 at 12:05 AM.