Bill,
If the engine has a pretty decent size cam it can cause the vacuum to be low. if the timing is to slow it can also cause the vacuum to be a bit low. 15 inches of vacuum really isn't that low. Vacuum is measured by inches, not by pounds. Typically, a good running engine will pull 17-19 inches of vacuum at idle so I wouldn't be too worried about it.
But, to check for leaks you can spray something like Gum Out carb and choke cleaner around the base of the carburetor and around the intake to see if the RPM changes. You might check the idle air mixtures to make sure they are adjusted correctly. Do you know how to do this? If not, get back to me, it is pretty simple.