When you worm your birds, you're not -preventing- them from getting worms... you're preventing them from getting a heavy enough load of worms to kill them. All of the wormers for things like roundworms and tapeworms and cap worms that I know of (and I work for a living amongst vets) do this. Wormers are typically poisons which wipe out the current load of worms your bird has (and if your bird has access to eating bugs or soil outdoors, you bird almost certainly has worms in some capacity) and forces colonies to start over or find a way to reinfect if they haven't left eggs behind.
So -of course- I worm my birds, but I don't vaccinate them. The reason we vaccinate children before they go to school is because we KNOW there are children they may be exposed to that may have things... which is why you would vaccinate your birds, if you knew there was something (or even thought there would be something) they would be exposed to in daily life that they could catch.