I agree. 15 days is only enough for one or at most two countries. It’s not worth the hassles and expense of visas and border crossings with so little time, and there’s plenty to see in one country in such a short span. Consider this the first of many trips in the region and see if there’s any way you can get more time. Some people like to rack up visas in their passport – I hope you’ll concentrate on having a good time instead.
I love Vietnam: rice fields with workers in conical hats, buffalos, bicycles, beautiful ladies and school girls in ao dais, long hair, sun hats and gloves, delicious food, etc, but Vietnam really requires a month – 15 days for the north, the same for the south. I prefer the north: beautiful Hanoi, the surreal landscape of Halong Bay, a train up to the hilltribes in the northern mountains, and if you’ve got time the great old trading port/beach town of Hoi An.
Or, for a very relaxing break, fly to Bangkok, take a train to Laos and explore that lazy, peaceful, undeveloped country with its myriad beautiful temples, monks and ladies in beautiful dresses. Take a bus up to the world heritage site of Luang Prabang with so many great temples (like Thai temples but less ostentatious) and good food beside the lazy Mekong river, then take a boat back. And, if you’ve never been to Bangkok, some people criticize it and say to get the hell out of there, but I like it: temples, water taxis, kick boxing, nightlife, etc.
Another lazy, peaceful country with fantastic temples (round golden spires) and the friendliest people in the world is Myanmar (formerly Burma). Although their government is terrible, illegimate and oppressive, the people are extremely kind and diverse and at the end of 15 days you’ll be wishing you could stay longer.
In 15 days you could explore Cambodia: Phnom Phen, a boat to Siem Riep and the amazing ancient kingdom of Angkor Wat, and beautiful, unpopulated beaches in the South.
Or, for an easier trip but rubbing elbows with more tourists, Thailand is great. Everyone who goes to Chiang Mai in the north goes on a 3-day trip riding elephants, hiking through jungle and riding bamboo rafts. That’s fun. Also good night market there, massages, cooking classes, etc. Other areas in the north also are great to explore and have fewer tourists. Or, you can take a train south from Bangkok and hang out in the islands, snorkeling, relaxing on the beach, motorbiking around the islands, eating good seafood, etc. And of course, Thailand has many very friendly, pretty girls in the north or south, if that matters to you. One good thing about Thailand, in addition to the beautiful scenery, delicious food and friendly people, is that it’s so easy. That’s why it has the most foreigners, which is its greatest drawback.