A "vaccine" is used to cure the people suffering from bat fever. Vaccines do not cure disease; they prevent the vaccinated from becoming infected in the first place. Injecting a vaccine into an already infected person will do nothing.
If Maggie knew what silver fulminate is then there is no way she wouldn't have known the barrel couldn't possibly have been filled with it. Silver fulminate is such an unstable explosive that it self-detonates under its own weight in extremely small amounts.