From BillH on another topic: Well, I have to say I have never been able to pull any more than 16 on the vacuum gauge and the wipers have never worked like I think they should.
16 is not super low, but... the mite should show 17-20 at idle. You can check on either sides at the center fitting on the valve covers.
In my experience, the most common cause of low vacuum is hose or fitting problems. That's the first thing to check.
Second is a carb or intake gasket leak. (Could be a crack in the manifold)
It's kind of messy, but you can spray WD40 around all the gaskets wile the engine is running. If the idle changes, you found a leak...
I've seen some mechanics use propane from an un-lit torch, but I prefer to clean up the mess from the WD40. (The engine probably needed a bath anyway!)
The other common cause is worn piston rings. A compression check will let you know for sure. A wet compression check will help narrow it down to rings/piston or head/valves.