Yes, 2 weeks would be just enough, something like this: 1 day to get to Nha Trang-HCMC, 4 days in/around HCMC including daytrip or overnight trip to Mekong Delta, 1 day to get to Phnom Penh (although it doesn't really take that long), 2 days PP, 4 days Siem Reap/Angkor. That's 12 days including intercity travel, leaving you 2 more to play around with, go to S'ville, etc. If you have satisfied your beach urges in Nha Trang, you may want to dispense with S'ville. Do some research on crossings Vietnam > Cambodia. While most people go through the main border crossing linking HCMC and PP, in the last few years, a number of other crossings have opened up that give you more direct link between Mekong Delta area and southeastern Cambodia--useful if you don't want to backtrack all the way to HCMC.