Cured Ditto Clear is stiffer than vulcanized natural or silicone
rubber. Don't make the mold any thicker than it needs to be.
Vacuum the mold only as long as the directions specify. You just
want to remove mixed-in or trapped air. If you keep the vacuum on as
long as you keep seeing new bubbles form you will be there forever. I
think Ditto 'boils', in the sense that bubbles will continue to form
deep in the mold as long as you vacuum, but once you turn the vacuum
off those bubbles shrink and disappear. If it bothers you to turn the
vacuum off while there is a large bubble clinging to the model, wait
until the bubble rises off the model and then turn the vacuum off.