Low Refrigerant- A/C off

Got this on my 01 Eldo.....one mechanic says must vacuum lines before adding freon ....his cost $120............another mechanic said vacuum not necessary & added 2 cans of freon but no cool air but low refrigerant-a/c on off disappeared from info bar......he said he would need to do more testing to determine problem w/o messing with trying to bypass ? (something) and fix it right so computer would be correct in reading going ahead.....thinking about leaving with him tomorrow...........anyone have opinions? they are very welcome.