Welcome to my side of the world! ;p Here's a blog I found really informative about cycle touring in the places you mentioned: http://www.crazyguyonabike.com/doc/inching.

If you want a sort of launch pad to go to various places from, I'd suggest Malaysia or Singapore. You'll find both places to be a good start point to being introduced to South-East Asia, plenty of English spoken and quite modern, also lots of budget flights to Cambodia, Laos, Vietnam.

If you do want to try cycling in Malaysia which, is beautiful, huge and amazing, have a looksee at my late friend's blog. He rode in Malaysia alot and did a 10 day solo tour from the south of Thailand to Singapore. Link is here

Either way, good luck and I hope things work out for you!!