I took a trip to Spain with a layover in the United States, so I processed the visa as a transit visa. I still have the visa valid, and this time I want to go to Miami for vacation. Do I need to re-process the ESTA?

Yes, you must re-process the ESTA. When filling out the ESTA visa form, we are asked for the stay information in the country. This information can be edited and changed if we travel to the United States to another city. But in the case of visas processed as transit visas, this stay information does not exist, and therefore we must re-process the ESTA visa.