My strategy would be get the Chase UR bonus points and transfer them to Korean Airlines so you can book a business class ticket. But $520 is a good price for a R/T with 1 stop, even though I am not a big fan of Air China and definitely don't recommend stopping in Beijing because it's a pain. I'm seeing ANA with a layover in Tokyo for about $610 (9/19-10/3), which would be more appealing to me.
As for hotels in Vietnam, I've stayed at some under $10 a night to over $100 a night. If you're staying in D1, then it's going to be more expensive. You're best bet would be independent hotels for about $25-50 a night. The major chains will be closer to the $100 range. You can also try AirBnB.
Two weeks will go by fast since travel time will be 2 days and then you have to deal with jet lag as well. It's also hot and humid there so most of the time you will just want to stay in your hotel during the day and sleep. 1 week has been my shortest trip and 3 months has been my longest.
I would recommend 3 weeks and use AirBnB for some nice places for under $30 a night.