I am trying to plan my first visit to Cambodia and Vietnam. I have 2 weeks, on the second half of July (in a month time!). I've been reducing the number of places to visit to "just" the south of Cambodia and Vietnam, as I prefer to know one place better than rush to many places and see/learn little from each. At the moment, I have the following on my draft:
Siem Reap (7 days)(includes 3 days for Angkor and 1 day for Tonle Sap Lake)
Phnom Penh (4 days)
Ho Chi Minh City (4 days)(includes 1 day for Cu Chi village and 1 day for the Mekong Delta)
My interest in mainly photography and culture/scenery/people, not interested in laying down in beaches... What do you think of the above plan? Any other suggestions? Also which area/hotels (3-4 stars) to stay in those cities, taking in consideration my interests?
You might subtract a day or 2 from your visits to Siem Reap and Phnom Penh. Because it seems like according to your travel philosophy, you won't have enough time for the Mekong delta. Better to stay at least overnight there.
Phnom Pehn is wonderful on the waterfront, and there is the obligatory (for many people) one day genocide tour. But most of the city isn't screaming for attention from tourists.
If photography is your interest then you should consider Kampot / Kep and surrounds with lots of interesting scenery. I found colonial architecture particularily impressive in Kep wandering around the old french quarter from one torched crumbling villa to another. You can explore Bokor national park, enjoy the peace and quiet of the Kampot river, the fish market in Kep, pepper farms, salt fields, caves - lots of photo opportunities. You can easily spend more than just a few days there.

Really depends on you, but HCMC is not such a nice place, I wouldn't spend 4 days there, the Mekong Delta is much nicer and one day there you'll see very little. 7 days in Siem Reap is also arguably too much given your short time frame, usually 3-4 days is enough, but again no one can decide for you. The genocide tours are by no means obligatory in Phnom Penh, so feel free to skip it. Kampot/Kep are too quiet for my taste.
Siem Reap is not in the south of cambodia (nit picking I know) and based on what you say then I would have thought too much big city. If it were me I would do 3 full days in and around Angkor Wat and a day to Tonle Sap Lake. I would at most spend two days in PP and then I would head to Kampot/Kep cross the border near Ha Tien and work my way through the delta to HCMC. I would stay in kampot rather than Kep myself but do not miss the seafood in kep
Thank you for the suggestions so far. It seems I would improve my plan by changing it to the following:
Siem Reap (6 days: includes 3 days for Angkor and 1 day for Tonle Sap Lake)
Phnom Penh (2 days)
Kampot (2 days)
? (1 day)
Ho Chi Minh City (4 days: includes 1 day for HCMC, 1 day for Cu Chi village and 2 days for the Mekong Delta)
Does this sound better? Or can we still improve it? Suggestions? What about travelling from one place to the other? And best areas to stay in those places?

If you ask me I would do 5 days in Siem Riep, 3 days in Angkor + 1 extra should be plenty. 2 days for Phnom Penh is good. You can fly Siem Reap to PP cheap on Bassaka Air or Bayon Airlines, from there you're going by land so you can leave it a bit flexible, 2 days still being very little for the Mekong Delta.
Does this draft plan look better:
Day 1: Fly from Kuching to Siem Reap, hotel check-in, Pub Street, Night Market, Old Market, Wat Preah Prom Rath
Day 2: Angkor Wat (Small Circuit): Angkor Wat, Angkor Thom, Bayon, Ta Prohm, Banteay Kdei, Sras Srang
Day 3: Angkor Wat (Grand Circuit): Preah Khan, Neak Pean, Ta Som, East Mebon, Pre Rup + Banteay Srei + Cambodia Landmine Museum
Day 4: Angkor Wat: ?
Day 5: Phnom Krom Pagoda + Tonle Sap Lake / Kampong Phluk Floating Village
Day 6: Siem Reap to Phnom Penh. Either fly or boat.
Day 7: Phnom Penh: Royal Palace, Silver Pagoda, Wat Phnom, Tuol Sleng Genocide Museum, Choeung-Ek, and the chilling Killing Fields. In the evening, wander along Sisowath Quay, the riverside esplanade.
Day 8: Phnom Penh: ?
Day 9: Kampot
Day 10: Kampot
Day 11: Go to Ho Chi Minh City, Hotel Check-in, HCMC sightseeing: Dong Khoi Street, Notre Dame cathedral, The War Remnants Museum, Reunification Palace
Day 12: HCMC: Cu Chi village, Tay Ninh/Cao Dai cathedral
Day 13: Mekong Delta
Day 14: Mekong Delta
Day 15: Mekong Delta
Day 16: fly to Bangkok and then London
How to go from PP to Kampot (bus? how many hours?) And how to go from Kampot to HCMC (bus? how many hours)? Where to stay in the Mekong Delta? And how many hours from the Mekong Delta to HCMC airport?
Phnom Penh (2 days)
Kampot (2 days)
? (1 day)
Ho Chi Minh City (4 days: includes 1 day for HCMC, 1 day for Cu Chi village and 2 days for the Mekong Delta)
Does this sound better?
Yes, this sounds better, but if you're going to Kampot, you should cross into Vietnam at the Ha Tien border, and spend your time in the Mekong Delta before going on to HCMC. The travel flows better, and at the end you'll be positioned to fly out of HCMC.

I agree with Mstep above, that you need to travel through the Mekong Delta, to get to HCMC and that is the final place in your itinerary.
I would consider something like the following:
Siem Reap (4 nights), Phnom Penh (2 nights), Kampot (2 nights), (Ha Tien - Border Crossing), Chau Doc (2 nights), Can Tho (2 nights), HCMC (3 nights)
If you leave out Kampot, you can go straight from PP to Chau Doc and adjust the times in other places to suit.
I really like Chau Doc - in particular the Nui Sam (mountain) nearby and Nui Sam village.
Chau Doc is a vibrant small city with not many tourists, a large local market, floating villages along the river etc.
Can Tho is a sensible place to take a half day small boat trip to the floating markets.