15 inches of vac at idle

Try running the compression test wet and dry back to back.

Test it dry and record the readings. (2 or 3 times is good).

Then get an oil can and squirt 2-3 shots of oil in the cylinder and rerun the test. The oil will help seal any leaks in the rings.

If the readings jump up with the wet test vs the dry test, then there is a piston/ring/bore sealing issue...

If the results stay pretty much the same, then if it is a low cylinder, the problem is in the valves, get a valve job....