Some form of getting the air out of any sealed cavity is a must, regardless if it is vacuum or pressure formed. Pressure forming could make the problem worse because you can develop greater positive pressure than you can get from vacuum verses ambient pressure. It also takes more structural strength in the machine and related tooling to keep the air pressure sealed.
Theoretically, with materials less than 1mm thick, the vacuum hole size should not be any larger than one material thickness that you are forming. Most of the time a much smaller hole works best, but you might need more of them. If the holes end up too large, I have drilled the hole out larger, then fill with epoxy and re-drill smaller vac holes or beat in a small aluminum rod/rivet/plug and re-drill as needed.
On thin materials, too much heat allows the material to pull into the holes easier too. Getting the material to just the right temperature can be merely seconds from too much or not enough heat.
Machines that can do pressure forming have an oven that the material slides into for the heat cycle, then slides out to the forming station. The forming station consists of an upper and lower cylinder mounted platen that can be moved up or down into position. While one platen holds the die and the other platen holds the seal ring pressure box. Normally the whole cycle is automated with timer relays for each portion of the total cycle sequence. The automation of the timed sequences maintains consistent duplication of the formed parts which would be harder on manually operated forming machines.
In some instances you can use course sand to sand blast the surface of aluminum dies. This helps retain an air gap on the die surface so it is less likely to seal off around vacuum holes.
Some forethought on the die design, if it is built up from aluminum plate, can have vacuum ported between the parts. In other words, larger vacuum holes can be hidden under another blocks mounting surface that has band sawed slot on its underside before it is fastened to the main die.
Unfortunately, it comes down to better equipment and/or methods that work around undesired results.