It is not. The mercury in (some, but not all or even most, these days) vaccines is ethyl mercury. Here is an article that explains a bit more of the similarity with dimethyl mercury, though - they are both organic mercurials:

>Both ethyl mercury and Hg2+ are very neuro toxic compounds. However, ethyl mercury is more rapidly partitioned into the hydrophobic (fatty) tissues of the central nervous system and is a more potent neuro toxin than Hg2+ based on this “partitioning factor”. It is this partitioning factor that makes organic mercurials such as dimethyl mercury so neuro toxically lethal (this is the compound that caused the death of a Dartmouth University chemistry professor after she was exposed to a drop or two on her gloved hand).

>The concern with organic mercurials, such as thimerosal, is that such compounds can be perceived as “pro toxicants” just as certain pharmaceuticals can be classified as “pro drugs”. This means that the original compound, e.g. thimerosal, is less reactive giving the compound time to partition into certain areas of the body before it breaks down releasing the ethyl mercury and then further releasing Hg2+. However, while attaching ethyl mercury to thiolsalicylate makes the ethyl mercury less reactive it most likely allows increased partitioning into the central nervous system before the ethyl mercury is released and thereby, increases the neuro toxicity per unit ethyl mercury involved

https://vaccinechoicecanada.com/vaccine-ingredients/thimeros...

No its not as you pointed out but didn’t clarify. Ethylmercury is not the same as methylmercury.

I would think it would go without saying vaccines don't use one of the most toxic substances known by virtue of the fact people aren't dropping dead.

Ethylmercury is used insted of (di-)methylmercury, because it is cleared from the body faster: https://www.cdc.gov/vaccinesafety/concerns/thimerosal/index....