hello ! lots of negativity here . awesome idea ! i think it might make the most sense for you to fly from vietnam to bangkok . if you want to see cambodia along the way , you could also fly to phnom penh and hit up siem reap , then head on to bangkok . it takes quite a while even by bustrain to travel around this area and nothing ever runs on time . i ' ve been traveling around thailand for the past month and even flying as much as possible is still quite time consuming and exhausting . within cities and countries you can rent bikes for really cheap , and you don ' t have to worry about the upkeep on them as long as you don ' t crash . honestly it would probably add up to cost about the same as purchasing a bike , and you get a nice new serviced one in every city ! AS FOR COOL SHIT TO DO ! khaosan road in bangkok is full of debauchery . go there . pai is an amazing little hippie mountain town worth going to great motorbike ride from chiang mai , you can rent bikes and they ' ll transport your stuff for you ! awesome party scene . tons of backpackers . so much fun . ko samui ! chaweng beach is dedicated to the party scene when the full moon party isn ' t happening , but if you can make it to the full moon party google for january dates , should be mid month , you can use ko samui as a jumping off point for ko phangan and go to that . i haven ' t been but it ' s supposed to be an amazing all night beach party . most hostels require a minimum five night stay so that days surrounding it are full of people trying to party . also near ko tao , an amazing place for diving ! chiang mai ! very cool city , way better than bangkok . much better food . this is where you go if you want to hangout with elephants and tigers . also take a cooking class here ! it ' s so much fun . no suggestions for new years but i know i ' ll be down near koh lanta by that time and hoping to find some fun . let me know what you find out ! as for hostels i would suggest happy house hostel if you make it to pai which you should do cuz it ' s honestly the coolest place i ' ve been to in thailand . in chiang mai try to be on the north east corner inside the moatold city , that ' s where all the backpackers stay . in bangkok definitely stay near khaosan road as you ' ll probably get shitfaced there and want to walk home . if you go to ko samui , i really liked casaluna !