When would I want to use the vaccinator, specifically over the default medigun?
The vaccinator has an almost negligible 10% resistance to one damage type during regular usage, which to me does not outweigh the slower overheal build.
During uber, you are only mostly resistant to one damage type, compared to completely immune to all damage types.
Another question already asked reveals the mechanics of the vaccinator.
When should I be switching to the Vaccinator? When is it tactically a better choice? I usually run medigun or kritz, and usually play in pubs.