Angkor is amazing, but Siem reap is very touristy and if you're going to do it i'd spend 3 or so days there – there's a lot to see, getting through the ruins can take time especially if there are a lot of tourists, and some of the ruins are pretty distant but well worth the visit, bantaei srei, beng maelia
Phnom Penh is not very beautiful, but it's less touristy than other capitals and there are a few very interesting things to see, mostly to do with the Khmer Rouge massacres. Also, if you're going to Saigon it's on the way.....boat or bus.
As for Thailand, Phuket has more to do (if you're not the sitting on the beach type) than the other tropical islands, but I think most of the other island have better beaches. If you're going from Bangkok to Chiang Mai or vice versa, you might like to stop along the way at either Ayutthaya or Sukhothai, both older capitals of Siam with decent ruins. I personally think Ayutthaya is better, and it's more convenient if you want to catch the train.
Vietnam has a lot: Hanoi is good, with Ha Long bay just up the road but be careful with who you book the trip with. If you wish to do the hill tribe thing outside of Thailand, Sapa is nice. To me Nimh Binh/Tam Croc is overrated, especially if you've seen Ha long bay. Hue, the old imperial capital in the centre and masoleums not far away is a good stop, Hoi An is quite beautiful with it's historic vientnamese and french colonial architecture (probably the best outside of luang prabang) but I probably wouldn't want to spend too long there. Nha Trang has nice beaches, and a few sites including some cham ruins (*if you've been to angkor already, you'll be bored). Saigon has loads to do, with a large variety of stuff you can do just outside of the city too. A trip to/through the mekong delta can be fun/interesting. Dalat is not really worth the effort to get there.
It really depends on how much time you have, where you should go – but your list looks pretty good at the moment.