A/C not holding vacuum?

With a leak that long I would do a system flush and replace the drier and ex-valve because air would have gotten in. As for the vacuum, yes not holding would mean it has a leak. The easiest way is to put pressure into the system and then use something like soapy water or other detection fluid and just look for the bubbles. That works for the items you can see, not good for the evaporator core unless you can get a camera in there along with a way to spray the core. For the lines and other pieces it works. I use nitrogen on systems that just started leaking, but if it has been setting a while I use CLEAN shop air, I picked up one of the oil free pancake compressors just for that. Added a filter to the outlet side just to keep water out as well. Then 90 psi or so works.