Since your first stop is Singapore - I would recommend getting visas for Cambodia and Vietnam there. I have gotten visas for both countries there, and it was relatively hassle-free. I had quite a bit of time on my hands, so I went with the standard processing, I think 2 business days for Cambodia and 5 business days for Vietnam. If you spend some $$, they'll speed up the process considerably.

Supposedly, Bangkok is also great for procuring visas, but I like the fact that Singapore allows you to get all of this done with people who speak decent English - not always a given in Thailand or elsewhere.