This used to be the deal in getting Vietnam visas in Cambodia:
- Phnom Penh, 3-5 working days; interestingly, it may be cheaper to use visa service than to go to the Embassy
- Battambang consulate, same day visa, open daily
- Sihanoukville consulate, same day visa, open daily
You can probably verify the latest at the
thorntree site.
If it was me, I'd rather hang around Sihanoukville of these 3 places.