I think your idea is mostly doable, but you may be wiped out and not leaving yourself enough time for travel. My gf and I had about 14 days over the winter and we did 3 countries -- thailand, cambodia and vietnam. We barely had enough time and only spent a day or two in each of the cities we visited (bangkok, siem reap, ho chi minh, hanoi and a beach town).
When you consider travel times in Cambodia and Laos, you are really pushing it. Esp. if you have some monsoons, you will be delayed for sure.
I'd figure out what you really want to see in each country and then see if you can't do the same things in one of the countries you def. want to see. Example: Say you want visit the floating villages in Vietnam. Well, Cambodia has some cool floating villages so maybe you could spend longer there and skip Vietnam. Etc. etc.
As far as border crossings, this you need to research too. If you arrive too late to a border crossing, it may be closed. Or, you may deal with huge lines that interrupt where you will end up for the night. Plan on hitting crossings as early as you can and plan that into your travel.
Oh, and travel into Vietnam overland requires a visa that you CANNOT get on the Internet. You have to apply and receive before you get there. See http://alexpickett.com/2011/03/04/alex-and-chickpea-do-southeast-asia-the-quest-for-the-elusive-vietnam-visa/
If you want some good advice about overland travel, check out this guy's site: http://www.talesofasia.com/
And you can check out mine too and look for the posts about SE Asia: http://alexpickett.com/category/alex-and-chickpea-do-korea/
Good luck! sounds like an awesome trip!