Generally the weather tends to consist of little rain with average temperatures of 19°C.

https://www.selectiveasia.com/vietnam-holidays/weather/december 

Money-wise, I would recommend that you bring what you'd be willing to spend (for example, enough money for the shops which sell silk clothing, silver products etc. on the Hanoi cyclo tour).

Please take a look at the below article for more information regarding currency in Vietnam:

https://www.tripsavvy.com/money-in-vietnam-1629945 

Kind Regards,

Amy