You've gotten some great replies already on here. We've spent lots of time traveling in SE Asia and the biggest recommendations I could give you are to skip the packaged tour and to slow down.
1. You can do four countries in 35 days but you'll be spending too much of your vacation dealing with transportation/logistics, SLOW DOWN and smell the Durian.
2. You don't need a packaged tour for any of those countries, they are heavily traveled and have an easy backpacker circuit that covers all of them.
Unless you absolutely must see Angkor Wat the country to take out (IMO) is Cambodia. Nothing against Cambodia it's always worth a visit but given your timeframe something should be cut. Vietnam/Laos have far better natural landscapes, Thailand/Laos have better food, you'll get enough culturally from a combination of Vietnam and any others since Vietnam is more culturally (and culinarily) distinct than the other three.
Everyone has their own preferences and travel style, but some comments I find dubious:
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Originally Posted by Florence44214
The best time to visit is in June, July as the climate would be neither too much hot nor rainy.
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Originally Posted by sponger42
Wear boots
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Originally Posted by sponger42
- There are no laundromats anywhere in SE asia, so you'll have to give your laundry over to locals who will do it overnight for a fee (and hopefully bring it back), or wash it yourself in the hotel sink or bathtub
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Quote:
Originally Posted by sponger42
Stick to cooked food.
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June/July is the rainy season in the countries you mentioned, November/December/January are drier and much cooler temperatures.
I don't know why you'd need to lug boots around, a pair of flip flops and some tennis shoes will work just fine just like all the locals wear.
There are plenty of laundromats in SE Asia, in some countries (like Singapore, Malaysia, and Taiwan) we used coin laundromats far more than laundry services. Point stands though, labor is so cheap in places most SE Asia that you can get it done as a service by the kilo. Good idea = spread everything out on the floor of your guesthouse and take a picture with your phone first, we've had stuff go missing but when we go back and zoom in to show them the exact thing their faces light up and they go right to the "where did this come from?" pile and hand it to us with apologies.
On the cooked food, peeled food is just as good you don't want to pass up on all the delicious precut papaya, pineapple, jackfruit, etc. that street vendors sell it's great for breakfast or a snack on a hot day. Also some of the best foods in the region are salads. A reputable place frequented by locals is a great chance to try things like glass noodle salad, papaya salad, etc. it's wonderful food that is quite emblematic of the region and it would be a shame to miss out.