I've done HK, Saigon and Bangkok in one trip(2 weeks). Loved that combination but we had actually been to Bkk before so we used that to relax. I have to say that I found KL to be disappointing(although we did have a great hotel there).If you don't fancy KL or an alternative, I would add a day to each of the three remaining. While in BKK have a look at doing a river Kwai trip. You can always stay overnight.