We eat about 50 mg of aluminum every day, while a vaccine contains less than 1 mg. About 0.3% of eaten aluminum is absorbed, so one vaccine shot equals about a week of eating, in terms of aluminum in the blood. Such low levels of aluminum are quickly filtered out by the kidneys, and aluminum toxicity is generally only seen in people with reduced renal function.
That fact usually causes the antivaxxers to declare that somehow, injected aluminum behaves differently than the aluminum absorbed by the GI tract. They can't explain why. They just have faith that it does. Sometimes the word "nanoparticles" is tossed out to confuse the issue.
So why aluminum in autistic brains? The best science indicates it's a result of the autism, not a cause.