In mid march is quite good time to travel in Vietnam. It is not so cold and not too  hot in the North of Vietnam but it is hot in the South of Vietnam. From Bangkok to Hanoi, you should take the flight with Air Asia. It has cheap flights from BKK to Hanoi. Please have look the link below  http://www.airasia.com/my/en/home.html. Other way, you can also check with Vietnam Airlines at http://www.vietnamairlines.com/vn/en/ From Hanoi to Ho Chi Minh, you can get the train if you have time then you can see great outdoor activities. Regarding the sightseeing in North of Vietnam, you should spend time to visit the highlight of the North including (Halong bay for 2days/1night. Sapa tour for 2days/3nights. The both itinerary can join with other people. The price for group tour is $ 110 for Sapa light trekking tour and $ 125 for 2days/1night itinerary. And in the South, you should visit Ho Ci Minh City and surround ( Mekong Delta full day tour, Cao Dai Temple and Cu Chi Tunnel. You can also join with other people. Hope it’ll help you.