Key to understanding vacuum is that it's not some magical thing. It's really just an area of low pressure. Air tries to flow from high pressure to low pressure. Vacuum doesn't "leak out", air leaks in or you're unable to establish a low pressure area.

You could put a check valve on the A/C system, but what would happen? The pump would establish a low pressure area on the "supply side" of the vacuum and the check valve would stay open - like it's supposed to in an "operating" system. The leak on the demand side would have the high pressure leaky air flowing into the low pressure pump so your system can't attain and maintain proper vacuum level (like it's not supposed to). So the check valve won't "protect" the vacuum level elsewhere.

The door lock reservoir checkvalve assumes a non-leaky system and holds the low pressure until you turn the key and let small bursts of higher pressure into the system. Lock and unlock a few times (or have leaky components) and the pressures equalize. The coupe seats are a one-shot deal.

A big hint if your seat backs are intact: can you see the locks rotate over, or do you get a screechy noise as the actuator compresses the spring? These are actually easy to check with a hand Mity-Vac. Pull the blue line under the seat, pull some vacuum and see if it holds. Don't forget there's a relay behind the glovebox. It's normally closed but is electrically opened by the door switches, so unless you've got a short in the door switches it'll be okay.