I've had very good reports of Bali, though I haven't been there myself. A colleague of mine got married there some years ago and told me he couldn't have chosen a better venue.
Thailand and Vietnam can be very wet - rainy and humid -in July and August. Some years torrential downpours for hours ,other years just a couple of hours in late afternoon.
Some visitors said it never bothered them ,others say it ruined their vacations.
Bali however is quite dry at this time and certainly is the place I'd go to .Unfortunately masses of Aussies think the same, so i'd avoid the crowded resorts in the south. Anywhere else is fine. Go google it now.
Bali however I don't include as part of SE aSIA
I've been in Thailand, Koh Samui, in July and it was fine. Just rained for one hour every second day. I think the western side of the peninsula (phuket etc) is a bit wetter at that time of year.
As has been mentioned it is rainy season in SE Asia, but those are the torrential showers that 20 minutes later have dried up totally.
My favourite place on the planet is Hoi An, Vietnam, we stayed in the Victoria Resort and loved it. Also did Hue (very historical but not very nice), Hanoi and Ho Chi Minh City on that trip.
In Thailand have been in Bangkok (loved it), Hua Hin (loved it), Pattaya (hated it) and Chiang Mai (loved it).
Bali is a great Honeymoon destination. Amazing accommodation for relatively lower prices when compared to other destinations. The Balinese people are very friendly and the service is great.
I'd recommend spending a few days in Ubud in the centre of the island, amazing place. Also have a few chill out days in one of the resorts down south..Seminyak, Sanur or Nusa Dua.
Stay away from Kuta. It's basically the Santa Ponza for Aussies.
If considering Malaysia for August, bear in mind Eid begins August 19th. As someone who honeymooned in Malaysia during Eid, I wouldn't recommend it. Locals tend to book into hotels to celebrate, and our experience was they liked to party in the rooms all day and all night. Same experience in 3 hotels (5*) so it wasn't an isolated incident over the week.