There are a seemingly endless number of places where vacuum leaks can occur. Ones that I've dealt with personally include:
- cracked air induction hoses
- leaking DISA valve gasket
- cracked vacuum hose (the 1/4" or so line that goes around the back of the valve cover to the ... umm ... aux air pump maybe? Can't remember right now.)
- cracked/crumbling oil separator valve hoses
- cracked plug on a vacuum nipple on the back of the intake manifold along the firewall
I'm sure there are more that I can't recall right now. Seems like every year or so I have to track down another one.
It can get rather expensive to replace things willy-nilly, though odds are you probably are in need of every one of these that you haven't already done. Instead of a smoke test, I set my OBD reader and laptop to show short-term fuel trim, then run an un-lit (!) propane torch at a low setting around the components on the intake side of the engine. When the fuel trim plummets, you've found your vacuum leak. Try not to blow up the garage while doing this.