H.S.
Honestly, you may never know if the use of the vacuum was totally necessary. But from what I understand, using the vacuum is not unheard of with c-sections...often the baby is bigger than anticipated and its just easier to do it that way then lengthen all the incisions that were already made. If you have concerns, then I would ask you doctor for an explanation so you can understand their resoning for using it when they did.
As far as calling a lawyer, unless your baby was damaged from the vacuum in some way, there is little chance that you would be able to make a case. Assuming your baby is fine now, I would get an explanation from your doctor and if you aren't comfortable with it then get a new doctor and move on.