Please be aware that Bangkok has multiple airports and that these results are for all airports serving the city.
The best month to fly between Bangkok and Hanoi is May and the most expensive month to fly is October. (Average prices, based on 2001 datapoints.)
The best day to arrive at Bangkok is Thursday and it would be best to avoid Friday, as prices are on average 9.6% higher than on Thursday. (Average prices, based on 1987 datapoints.)
The cheapest airline flying from Bangkok to Hanoi is VietJet Air. They are 49% cheaper than Vietnam Airlines. (Average prices, based on 2006 datapoints.)
The best time to fly is at around 6. (Average prices, based on 3422 datapoints.)