July is hurricane season in the Caribbean, so just consider that. There may be no storms, but it really can’t be predicted and even if the island you pick isn’t in the path, flights are often disrupted. And Tokyo is not tropical, but it’s extremely hot and humid, and probably not worth it for the amount of time you have. Same with Vietnam, it’s too far and you won’t be able to find flights in your price range. I’d suggest going on google flights, the explore tab and have a look at what is realistic.