That's a lot of miles to cover in three weeks.
I suppose you are using a combination of cheap airfares and overland travel. I don't want to put a downer on what you are trying to do but you will lose a lot of time fiddling around. for example although BKK- SGN is only an hour or so flight, unless you are travelling carry on only and don't need to be early to check luggage, given the amount of traffic jams in BKK, you have to allow a couple of hours pre departure. Then at SGN you will lose at least an hour at the airport if you are lucky, then depending on the hour of the day, taxi to town could take 3/4 hour so basically you will have blown a day.
I could say the same about PNH to Sooky and then REP whether you fly, bus or taxi. But if you go by road, these can be long, stressful, boring and dangerous drives that often will take hours and hours longer than you may have budgeted timewise.