Jump up a week in Vietnam, you will cross border to Cambodia for next destination. Should arrive Hanoi first, then fly out from Hochiminh. Alternative choice is to cruise from Hochiminh to Phnom Penh, and enjoy your stay in Cambodia for approx 5 days before fly out.
BKK is the usual starting point form most travelers.
a) BKK take day trips to Kanchanaburi and Ayutulla .
b) Coach/bus to C.Mai. In C.Mai do a 2/3 days trip to Pai. Then go back to C.Mai for day trips to C.Rai and the Northern Thailand attractions ( C. Mai > 7 days ).
c) Krabi is way down South and famous for its beach. So it is a big detour if you want to go to Krabi.
a) Back to BKK by Bus/coach.
b) Then Bus from BKK to Siem Reap (Angkor Wat)
c) Then from Siem Reap to Phenom Phen by River(Boat) or Bus.
a) P.Phen to Ho Chin Min City by bus.
b) Then by bus following the route up to Hanoi. Visit interesting places as you head up north.
c) Lots of Beaches along the way up North. Danang and Halong bay are worth visiting.
a) Hanoi to Luang Prabang by bus/coach.
b) L.Prabang to V.Vieng by bus/coach.
c) V.Ving to Vientaine by bus/coach.
a) from Vientiane you can fly to BKK, or
b) Cross back to Thailand to Nong Khai ( Taxi or Bus ) and stay for 1/2 days.
c) Then from Nong Khai back to BKK by Bus or N.Khai to Loei to BKK. Depending if you have time.
Hope this helps. Have an Enjoyable ans Safe trip.
Good last post.
I would just add that Pai on a bus is a hard journey, and will take around 7 hours or so, its a good base for trekking in that area, but not sure if its a better base than CM. If you have the time then visit after CM. Chiang Rai is good, but not so much to see.
Also, if you are considering re-entering Thailand from Cambodia/Vietnam overland you will only get a 15 day "visa on arrival", meaning you MUST leave the country within that time. When you arrive by international flight into BKK you will receive 30 day "visa exemption", so doing the tour based upon the previous suggestion would give you a minimum of 30 days & a maximum of 45 days in Thailand.
You're going to see some great places and i've recently done most of your trip apart from Vietnam. I think you will find it hard to fit in Krabi without flying because your going to wanting ot head north from Bangkok. Chang Mai to Pai is easy and you can get transport to the border to Laos from Pai as well.
Getting from Thailand to Siam Reap will take a good days travel and i found that getting out of Siam reap to Vien Vang could only be done with morning ealry afternoon buses. With the road ( thats getting quite good now ) not being travelled ot much at night. Im also not to sure if you can get a bus into Vietnam from Vein Vang without stopping off in Vientiane.
It can be done but you may find it a rush at some points. I also think that you should make sure you sort out transport as soon as you get into a new place. Just incase you need to wait an extra day.
Have a super time