15 inches of vac at idle

15" vacuum does not seem abnormal for a 268/272 duration cam...?? Any symptoms of poor running? Any excess oil consumption? How many miles on the engine, and are they street miles or ??

Repeat the compression readings with carb propped open and all plugs out; make sure the engine cranks over the same 4 or 5 times per cylinder reading. Make sure the threads on the tester seal well each time. If still low, add a teaspoon of motor oil to the low cylinder(s) and repeat test and record the readings. If the readings increase then ring, piston, or bore issues are likely. If no increase, then it could still be rings (i.e. bad top, 2nd AND oil rings, letting the cylinder oil kinda/sorta seal up the top rings all the time), but it is becoming more likely valves or guides: worn, sticking, etc. Put compressed air in each cylinder at a moderate pressure and listen for where the leaks are.