If the panels are only leaking when they are first turned on, then it could be the VRV. Sometimes they will let a bit of water through before shutting off. However, if you still see the water dripping after several hours of operation, then it is likely a very small leak somewhere. Just trace it back to it's origin to make sure.

As for priming the panels. After all the air as stopped coming out of the returns with the bypass open (i.e. bypassing water around the panels), if you then close the bypass, is there any more air that is forced out of the panels? If not, then they are probably fully priming even when the bypass is on. However, if you get more air out, then the panels may not be fully priming.