Did you actually see your ball python vomit? From the description it sounds more like diarrhea than vomit.
How much of 'vomit' is there? Just a little spot, like a teaspoon or two, or is there like half a cup or more? When was the last time you fed him and he ate? Typically if fed within a week or two, any vomit would contain the undigested remains of the last meal, more than just yellow liquid.
Though if he is vomiting alot or having really runny stools, both are signs of internal problems, which to me, means immediate vet trip. Excessive vomiting could be problems with the stomach, throat, etc, or environmental factors (ie if its too hot in the cage...heat stress can cause vomiting). Liquid poo often is a sign of internal parasites, so bringing in a sample of the vomit or poo, whatever it turns out to be, to your vet would be a good idea too.
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PHLdyPayne