With all this discussion I searched for "external catheter" and have a feel for what is going on.
Unfortunately I don't think stopping the vacuum until urine is detected in the tube will work.
My understanding is that the "wick" of the urine pickup is under vacuum all the time. When urine is released the existing vacuum will suck the urine into the container.
If the pump we to be stopped, when urine was released it would just sit in the wick and leak our around the wick assembly. There would be no force to move the urine through the wick for sensing further on.
I do not know the possibility of sensing temperature at the wick for pump control. If somehow the wick saturation level could be determined the I believe controlling the pump would be possible.