I will try the 7mm wrench next time. Thanks.

I was able to push two thin strips of aluminum (cut from a can of sardines - don't laugh) into two of the slots and uncouple the vacuum pipe connection. It took me a good part of an hour trying various tricks. I didn't want to break anything.

I'm astonished that there is no special tool for this. If technicians had to spend so much time disconnecting vacuum lines they would have no time for anything else.