What kind of plastic tubes are you using? I use the older Q-Tip tubes (found in bargain stores), but I have found that not all hollow q-tips are created equal, some of the "no-name" ones are very thin walled and collapse under even medium thread pressure. The thicker tubes will take quite a bit more pressure but even those will be a little tighter on the pull off the mandrill.
I have a Renzetti tube fly vise - and I love it. The tubes do not slip and the rotary action is wonderful, plus I went and bought the Traveller head for the vise and now have both. Those who already have a Renzetti Traveller Vise should know that you can order the tube fly head which is considerably cheaper than doing it the other way round - like I did!
Tight lines - tyler.