I'm one of the "crazy, irresponsible, selfish" moms who chooses to not vaccinate my son. I've read too much information, not just online (in fact, very LITTLE of what I know comes from online sources) but also from doctors, the PDR, the NHO, the CDC and the Useless FDA! Enough about that. I'm sorry that your lil girl had to go through that..
To answer your questions... YES it is ENTIRELY possible for your son to have mumps despite having the vaccine. Vaccines do NOT convey immunity. In fact, the problem is that the vaccines are useless against the mutating viruses. All vaccines were created at the time when the illnesses were already on their way out. They are all very dangerous and have serious complications. Mumps can be treated... and he will survive and the greatest thing... he will have LIFETIME immunity! He won't get it again. (Unless it's a very mild case which the vaccines sometimes produce... not a good thing because then he can get it again).
I have a great book called "The Sanctity of Human Blood" by Dr Tim O'Shea. Read it! He quotes the very people who create vaccines and market them. Also, he cites the PDR... crack open that book and I promise you that you will never think twice about vaccinating or not.
I'm sorry that you were attacked by someone else... I get it often so I know what it feels like. Most of the time I feel like asking that parent who is doing the attacking How SHE could risk killing her child to only possibly prevent an illness that is easily treated...
Feel free to email me if you need any encouragement or have any questions. :)