Cambodia you can do at the airport. The last couple of times I have done it, it’s been quick and easy.
Vietnam I always get from the Embassy in NZ before I go. The on line ones are only half the story – you still need to hand in the letter when you arrive, pay more money and then wait while they put the thing in the passport.
IME this can take a very long time. I once arrived in SGN around midnight and had to wait 90 minutes whilst they put the visa in because there was a massive queue. I had visions of my bag going around and around on the carousel and then just getting swiped by some chancer out to get the foreign devils.
Now, I get it before I go. Costs a bit more (got to keep the embassy beer fund topped up…) but saves a heap of hassle at the arrival end. You just head straight to passport control and out, whilst everyone else is waiting for their visas.