Southeast Asia Tour in June/July

My fiancé and I have always wanted to travel to Thailand and Vietnam. It has been on our bucket list since we met, so what better way to spend our honeymoon than to spend it traveling there? My concern is, I don’t know what the weather will be like, and I don’t know how much time we will need to spend in each place. We are hoping to stay in Hanoi, Chiang Mai, and possibly Siem Reap each for a few days. Any suggestions or input on how to go about these travel plans? Is two weeks enough time to travel through three different countries?