I understand that this is quite an open question but any ideas would be appreciated. I've done a good bit of traveling a few years ago and I've managed to get some time off work and persuaded my gf to come along with me. I've been to Thailand, Cambodia and Malaysia before however my knowledge of other locations in SE Asia is pretty limited.
We'll be arriving into either Singapore or Hong Kong in early July and we'll have a month. I'm thinking Borneo to see the wild life and Bali, would we be able to fit a few places in Vietnam in there as well? We're keen to see as many things as possible but then again we don't fancy having to move around a lot.
How about China? It is hot then, but you expect hot in much of Asia. That time of year, it rains about half the days in a month but that will be the same all around Asia. China is a big country with a lot to see and easy to enter from Hong Kong.
Distances are great so many choose flying as often it is little more expensive than an endless train journey.
Alternatively you could fly from Singapore to Medan and travel overland (and ferry) through Sumatra and Java to Bali.
I think you have to figure out what you really want. This question is so generally and hard to answer , Vietnam is a long beauty country, You have to take at least 1 month to explore all the the country. However there are some highlight places that you can consider are Ha Long Bay, Sapa (North), Hoi An, HUe (Middle), Phu Quoc, Mui Ne (south)
What ever you choose, Happy travels
Like everyone else says, it's kind of hard to answer without knowing what you want of this trip. But since you've been there before and mention Borneo, which is a bit off the usual tourist path, why not go for locations that backpackers don't traditionally visit? Some ideas, Sumatra, different areas of the Philippines, etc. There is so much to see in SE Asia. I don't want to say I regret going back to Thailand for another month the second time around, but I do sort of wish I visited some place different. Be creative