[Old Tele man]

...a simple vacuum switch "Tee'ed" into the intake manifold should work, with the switch set to OPEN when the vacuum drops below a preset value (about 10-14 inches Hg), and being wired into the hot line to the A/C compressor clutch.

...thus, when the manifold vacuum is HIGH (idle, light cruise, etc.) the switch is CLOSED and the A/C compressor clutch will operate as normal. But, when the manifold vacuum is LOW (accelerating or heavy load (hills), etc.) the switch will OPEN and the A/C compressor clutch will be disabled (only the compressor is disabled, all other fans/etc. continue working as before).

...might want to look into also installing an suitably size RC-snubber circuit across the vacuum switch contacts, as the inductive "kick" from the A/C compressor can/will eventually "eat" the vacuum switch contacts.