Mr Tilt,
Here's a thread that I started a couple of years ago when I was a new VED user.
Helpful Fall Weather Tip for VED users on HTSome of the comments seem to have been edited out. Not sure why. I don't seem to remember any problems with the thread.
It talks about
the problem of scrotal tissues being drawn into the tube. I think for most guys that can be avoided by pumping slowly and giving the tissues a chance to expand at their own rate but for guys with so-so arterial flow who need more vacuum it can be a vexing problem.