Best route to travel for visa purposes: laos to cambodia to Vietnam

I am planning to travel from january 16 to March 13 to Laos then onto Cambodia and onto Vietnam. I am an American. What is the best way to get into Laos and out of Vietnam (that will be the cheapest and as visa hassle free as possible)? My plan was to fly into Bangkok, overnight train to Vientiane, bus to LP, then bus down to Siem Reap, PP, enter into HCMC on Feb 14 (for Tet and to start the time allottment for a one month visa stay in VN) and make my way up to Hanoi before flying back to USA on March 13. The reason why i am flying into bangkok is for a cheaper flight AND most importantly, because I am thinking i can only enter VN once. Please help me understand:
1. Can i fly into Hanoi and bus to LP Laos, bus down south to PP and re-enter HCMC again with my same visa?
2. Is a multiple entry visa into VN much more expensive (please state price) than a single entry visa?
3. Any suggestions of a better route of entry and route of travel to visit the 3 countries?
I wanted to be in VN during tet (feb. 14) and to save hanoi and Sapa as my last visit as the weather will be better in March. Feel free to send me a private message or to make suggestions as to what city i should fly into? Thank you.