Josh just buy a 50l rucksack!
You will need, rain coat (when it rains there, well see my thread here:
http://forums.hexus.net/photography-...-flooding.html ), good pair of shorts for warm weather, trousers for cold, if you go to say Sapa or Dalat you will want stuff for 5 degrees. So you will be needing a mix of shirts and trousers. If you are short it is easy to find lots of stuff that will fit but cost nothing out there, if you aren't short then it's only t-shirts you'll find.
If you've got say 4 days worth, plus undies, some sandles for the hot bit, but some boots for trecking, don't forget your aid kit (anti-malarials, emergency water sterilisation etc) then add in some useful stuff, little bit of paracord and duck tape, a silk sleeping bag liner, a sleeping bag. Don't forget nets. Or just a shedload of DEET.
Then going away you've space for water (and beer!). I was in Costworlds in Picadilly when a BBC nature presenter (I forget her name) came in once helping a friend buy some boots and a bag. She helped convince him that your boots are the most important item, then your bag (I had been preaching this for a while) she was also suggesting buying a tent as you never know when you'll want to go impulse camping.... Yeah I'm not that far gone yet. But think about it, the chargers for everything, a book or even just a kindle. It all adds up very quickly. A good rucksack will make it so much easier on holiday.