The form of mercury in vaccines is
ethylmercury (aka: thiomersal, or thimerosal) which is
not known to bioaccumulate in any substantial way.
The form of mercury in tuna fish is
methylmercury which
does bioaccumulate.
In a vaccine, you get around 10 to 25 mcg of ethylmercury. In a can of tuna, you get around 40 mcg of the methylmercury — and methylmercury is very well absorbed in the gut (95% is absorbed).
So
eating a can of tuna not only gives you a higher dose of mercury than a vaccine injection, but the mercury in tuna is the bioaccumulating kind that stays in your body for long periods.
Other fishes with high mercury content detailed in
this post.
Furthermore, people with 8 amalgam filings in their mouth will absorb up to 70 mcg of elemental mercury each day (see
this post), some of which is converted to methylmercury by gut bacteria. So the amount of extra mercury you get from a vaccine (10 to 25 mcg) is totally negligible compared to the amount you get every day from amalgam filings.