Depends on what you’re going for. I been to all those except Vietnam. Completely different countries. From price, to infrastructure and weather. Personal ideas of a great vacation are very important to consider before saying go here.
Vietnam would be mine because I haven’t been there. Personally my list of countries I been to are
Japan-expensive but not too bad since the yen isn’t doing too well. Very easy to get anywhere and people are very nice and chill. Definitely the top if you want creature comforts but still feel that intense culture shock of an Asian country.
Greece-Cheap for Europe. Tons of character and historical sites. Beautiful, but a bit dirty. That didnt bother me though. This is a great place to feel history in my streets but not a place that doesn’t feel somewhat modern.
Thailand-Cheap, but the trip is a devil. Force yourself to stay awake to avoid some jet lag or you will waste a day or so being very sluggish. Very very very hot and sticky. You will be a sweaty pig no matter what. Bangkok is a hellscape imo, and you get bombarded at almost every turn by what some would call “degenerates”… Nice if you’re a young guy, but I would go to the islands in the south again instead. Stay away from the border with Cambodia, so only southwest islands.
Italy-beautiful, but can be pricey. Great people who will absolutely chat with you. Very gregarious people like us Americans in a way. Food is hard to beat. You can take a ship and cruise around the Mediterranean too, which is simply stunning as you go east towards Greece.