It's unconstitutional for electors to cast a vote for both a President and Vice President from the same state as themselves. Trump and DeSantis are both from Florida (Trump hasn't been a resident of NY for years and he certainly isn't going back now that they are trying to take his business from him through the courts), so the only way DeSantis gets VP is if Trump's margin of victory is high enough to ignore Florida, or congress picks DeSantis in an election where no-one gets the necessary electoral votes. Relying on either of those is dumb when you can just pick someone else as VP and give DeSantis some other slot in the cabinet.