Don't think it actually works like that.
Vaccines don't shield the system from infections, they prepare them from it. (Either through the injection of an infection, or the immune cells itself, or a related virus). Even if you're vaccinated, your immune system still reacts and has to fight of the infection. Besides that, there're thousands of minor viri, bacteria and other infectious agents which are found pretty much everywhere, and from which the immune system has to defend itself constantly.
You'd have to live in a bubble to get underexposure.
Edit: Anybody has a ninja vaccine.
The immune system basically just has a collection of antibodies it shoots at things to kill them. Getting a vaccine makes your immune system gain new types of antibodies, improving its arsenal. There is no downside to vaccines, barring gaining a (extremely weakened) infection of what you're trying to vaccinate against and things like irritating those allergic to eggs (since those are used to produce vaccines).
I don't know if it applies to vaccines (probably not), but overexposure to certain pathogens/allergens/poisons*, can result in an overreaction** from the immune system.
* Beekeepers are vulnerable to this. They can get stung so much that their immune system goes overkill whenever it detects the poison, doing more damage than it could've ever done.
**With deadly results