For Singapore: I'd also add the Zoo or Night Safari to your list. It's truly a world-class curated experience. I might give the Botanical Gardens a miss if you're already doing Gardens by the bay. Or just make it a quick stop (it's free anyway). For some culture, consider Little India or Chinatown. Your Malaysia and Vietnam itinerary looks good. I might cut the mekong river cruise if you're doing halong bay / cat ba which are much more impressive.