I used a compressor tank as a vacuum tank; actually I converted a compressor into a vacuum pump and it works very well.
I have seen some people claim that it is possible to implode a compressor tank by putting it under vacuum but obviously mine is okay to 26"Hg and a full vacuum is only about 30".
The tank has two advantages: One is that your pump can pull the tank down to a good vacuum and you can have a large diameter ball valve between the tank and your vacuum plate. This allows you to apply a large vacuum quickly and that sometimes helps snap the part down tightly and compress all the sealing strips. Sometimes if you apply the vacuum slowly just by turning the pump on the part never pulls down enough to form a seal so the pump can never develop a vacuum. Another advantage is that the tank can act as a trap so that if you get coolant into the system at the vacuum plate it does not get into the pump. Acually on my setup I had a coolant trap ahead of the tank as well.