I have been to all those places, and without looking at that link I'll tell you the places you need to visit.
Thailand:
- Bangkok (Bridge over River Kwai, floating markets, Khoa San Road, Grand Palace)
- Phuket (Patong - honestly so much fun - great night-life, lots of people, relaxing resorts, shopping)
- Phi Phi Island
Cambodia:
- Siem Reap (Angkor Wat)
Probably one of the most amazing things I have seen in my life. Make sure you have a good guide who can tell you interesting things about it and show you around to all the best temples, as it's very big - not just Angkor Wat but all of the smaller temples in its vicinity. Also, go at sunrise, as it's beautiful seeing the sun rising over the temple, there is also less people and it's much cooler. It gets very hot. I've heard sunset is also nice, but I only went for sunrise and day-time. We went to Ankor Wat, Ta Promh and Bayon (these three temples took all day and we went back the next morning for sunrise).
In Siem Reap, also go to Pub Street. Great vibe, lots of awesome restaurants and 50c beers!
- Phnom Penh
Killing Fields (gruesome, awful, but interesting to understand Cambodia's history with the Khmer Rouge and Pol Pot - after all their adversity, the Cambodians are lovely people and usually always have a smile on their face)
the Grand Palace is not as grand as Bankok's.
I found it quite dirty to be honest, but it was alright for two nights. Make sure you go to the Foreign Correspondents Club (FCC) - great food and $7.50 jugs of mojitos!
Sihanoukville (affectionately known as Snooki)
Haven't been, but am going back this year. I've heard it's awesome, and is on its way to becoming a big hot spot for tourists (so hurry before the commercialism destroys some of its charm)
Vietnam
- Ho Chi Min City
BUSY BUSY BUSY. It's an insane site. Motorbikes everywhere. It amazed me. Crossing the road is quite a challenge - but you just step out onto the busy road and walk straight and they literally just drive around you.
the Cu Chi tunnels are also very interesting. All about the Vietnam War - I even got to shoot an AK47 and machine gun.
Ben Thanh Market is good also.
Mekong Delta (what an experience. I would recommend not swimming in that water! We saw a floating television and a dead dog, not to mention all the other unseen stuff, The people are incredibly lovely though.
- Hoi An
by far my favourite part of Vietnam. Quaint, and French-influenced, Hoi An is beautiful. Lanterns are everywhere at night, making it gorgeous and there is so much nice food. We did a cooking course there called the Red Bridge something and it was good. We also visited some ancient ruined temples (although, honestly they have nothing on Angkor Wat).
We are going back this year to Hanoi and Halong bay and Sapa.
Your trip looks great! I hope you have a great time. I am going back to South-East Asia for my 6th time this year that's how much me and my family love it. If you've never been then it can be quite a shock (especially the busy-ness of Ho Chi Minh, but if you get past that and the beggars - there are a lot of poor people in these countries that sometimes don't have arms or legs and it can be quite hard to see - then it's great and has a lot of charm).
As long as you see Angkor Wat, Hoi An, and Halong Bay (and I'd suggest Sapa - its so beautiful and unique, or so I've read... I am so excited to go there) you'll have a fantastic trip. Make sure you research everywhere you go in depth so you fully enjoy the experience!