Would brushing your teeth immediately after vomitting cause tooth decay?

No. Please do: Gastric contents are very damaging to teeth. Brushing after vomiting removes stomach acid and helps prevent erosion caused by vomiting.

Rinse with flouride.: Acid can weaken enamel. Rinse mouth with 1/2-1 teaspoon of baking soda and water to neutralize the acid or rinse with flouride mouthwash, e.g. Act; if you can be able to eat after 30 min to replenish what you lost, brush using toothpaste with baking soda on a soft toothbrush right after. I hope this helps...

Better suggestion: Recently, it has been shown that it is better to wait 30 minutes after ingesting acidic liquids such as many sports drinks and some fruit juices before brushing your teeth. This leads to tooth erosions. With this in mind, i would suggest that you thoroughly wash your mouth out with plain water, and then you can brush and floss 30 minutes later.

Not really: Not tooth decay. However you may increase damage to your teeth by brushing weakened enamel. Instead of brushing rinse with water or better yet baking soda as it is alkaline and will help neutralize the acid. Another good idea is to use mi paste which is a very effective reminerilizing agent available form your dentist.