If you happen to come to Chiang Mai I will exchange all of it to baht at the rate of 230 kip for 1 baht.
It's certainly doable. Why not just reverse the order of the trip, and go to Cambodia first, then Vietnam south to north? The advantage of this is you'd be in Chiang Mai for Songkran, which would definitely be preferable to Bangkok. Also, the slow boat trip on the Mekong river is more enjoyable and less crowded if you start from Luang Prabang and head upriver to Huay Xai, because the boats coming from Huay Xai are usually packed full.
Your budget looks good. If you're careful with the money, you might even have enough for a luxury splashout when you get to Bali. I'd just go directly to Ubud from the airport, base yourselves there, and skip the southern Bali beaches. Bali is best for exploring culture and countryside (not beaches), and Ubud is the place to start.
$240 for 2 people !! - Alarm bells are ringing very loudly! I can't possibly imagine what you could possibly be getting for that sort of money!
Looks like you get royally scr~wed for that kind of money. Immigration is usually very quick, and waiting for your checked bag takes longer so travel light and do it yourself. If your flight arrives on time you might make it, and if you want to be 100% sure then change to a later flight in myanmar, should be easy. Mail them.
Just got back from Muang Khua , Muang Ngoi and Nong Khiaw (two weeks in the area)
Minibus from Luang Parabang to NK takes four hours and most leave at 09:00.
Boats between NK and MN leave at 11:00 and 14:00
Boats between MN and NK leave at 09:30 as does the one to MK
Boats between MK and MN leave at 09:30.
Boats and prices depends on numbers of passengers. For example the MN - MK boat costs 1,000,000 kip and they just divide it by he number of passengers, however you will never pay less that 100,000.

My friend and I are travelling in June to Chiang mai (3-4 days) then 2.5 weeks in Myanmar. Would you suggest flying to Yangon or Myanmar from Chiang mai. We leave from Bangkok. Unfortunately we have to go at this time and realise it will be difficult to get around with the monsoon but any ideas on itinerary would be appreciated and ideas for travel options and best type of accommodation to stay in. Thanks in advance

And just so readers who may only see the Thai entry information here are fully informed, that visa-exempt/visa-free, 30-day entry "extension"/"renewal" costs 1900 baht, or about US$57, as I recall; whereas a land entry/reentry for those eligible is free, but limited to two per year, as chris1965 noted.
My personal experience in November-December, 2017, was obtaining the free entry at Don Mueang Airport, an extension/renewal in-country in Chiang Mai, then a cross-border-and-return new 30-day entry at Mae Sai. (Note: there is a sign at that Thai border that says something like "No visa runs." And a young Russian traveler I met said he thought it was only that he showed proof of a flight out of Thailand that they had let him cross the border to do the visa-run - and only after much drama since, he lamented, he had told officials he was doing a visa-run. For me, I had originally expected to stay across the border for some time, and perhaps that influenced the officials to allow what turned out to be a visa-run. I was not asked to show funds or proof of onward transportation on my return.)

With apologies for the 'basic'-ness. I am headed Cambodia/Laos for 3 weeks in October. I will read all the postings and my answers may already be here:
1) Internal flights - the national airlines look expensive. Also I see that Air Cambodia has very limited hold allowance (looks like cheaper flight + pay for luggage). I gather there are other airlines which come and go. Should I try and book ahead or just go to an agent when I arrive Phnom Penh?
2) Nothing in the guide books but my reading on this site is that you can travel overland from Cambodia to Laos with ease. Is that correct?
3) Visa question - I fly in/out Phnom Penh at start/end. I plan to tour Cambodia and then head north, returning to PP from either Luang Prabang or Vientiane (depending on flights). Will I need to buy a second visa on returning to PP or does the 30 days allow you to come and go?
4) Sihanoukville reads as pretty horrible - with nice islands though (+ Kampot, Kep en route). The North East looks as if there is limited interest for someone that doesn't have days to go trekking etc. Kratie, in the East, seems to be dolphins, dolphins and dolphins (nothing wrong with dolphins, but I can see those in many places).With my timescale, I think I either travel West of PP up to Battambang/Siem Riep or up the Mekong side to Siem Riep. Any views?
5) Any views on the train service to Kampot/Sihanoukville?
Thanks, in anticipation, for your replies.
Here are some suggestions for your visit -
http://myaltruistictravels.com/2016/02/24-hours-in-luang-prabang/
Hello,
We are trying to arrange a SE Asian trip for 5 weeks ( 27 Dec - 31 Jan) and I have the habit of trying to cram in too many destinations. I would appreciate help in crafting an itinerary that allows a focus on a few places with more time to enjoy/relax. The kids (10 and 9 yrs) love water, animals, riding bikes; paddling canoes and exploring smaller quieter places. We like this too but also like temples and local markets. Hence why we thought Laos and Cambodia would be main destinations.
- Cambodia: We could fly into Siem Reap then bus to Battambang and out of Phnom Penh or Sihanoukville.
- Laos: fly into Luang Prabang, mini bus to Van Vieng and then to Vientiane before flying out.
- Transport: Fly between countries with low cost airlines where we can and within countries mini bus, boat, taxi.
- Flying in - to be determined but likely Singapore or Johor Bahru so we can take kids to water and theme parks.
- Flying out - this flight is booked departing Kuala Lumpur ( we can't change this).
Beach R & R at end of trip (7+ days):
Which beach destination? Cambodia, Thailand, Malaysia...or...?, with a relaxed environment and:
* good value accommodation (between US $25 and $80 pn]; * on the beach or walking distance to beach with a pool; * walking distance to restaurants; apartment or bungalow preferred but a good kid friendly resort can also be good; * an area where kids can play on sandy beach and options for the family to walk and explore or kyak etc; * away from mega built up & seedy places but close enough to a town where day tours can be booked.
We would appreciate any itinerary/destination advice and suggestions of places to stay/visit; length of stay or any other countries. Thanks.