It can be done, but it is not allowed on many airlines. If you are careful, no one will even notice.
vape, then blow your vapor into a napkin and nothing will be seen. Or, hold it a bit before the exhale and little or nothing will be visible. Or, vape, then blow it down towards the floor, and probably no one will notice. And, I have done it in the rest room on airplanes, but be careful, as I have heard that it can trigger the smoke alarm. Hasn't happened to me, but I have exhaled toward the floor. Be aware that Flight Attendents have had training on what to watch for, so they are looking, and watch out for them. Also other passengers - if they see vapor may complain to the staff. So, if you are careful, you should be OK! I have taken quite a few flights with no problems observing these precautions.