As most above have said really......
From personal experience using my own vac former for the first time at the weekend ..... I only dried mine in the oven at 120 deg c for 1 hour and it pulled fine , no bubbles at all.
First try was for 40 secs and the lexan wasn't quite soft enough to form properly, second pull was for 50 secs and was perfect
It'll also help from if you warm the mould up a bit first if you only doing a few ...
HTH