I did BKK in Club last year (in order to get to Vietnam too) and it was £500-ish in fees and charges each. Just like you, wanting to use up a 2-41 and a pile of Avios. Vietnam was fantastic— hope you get you trip sorted and have a ball.
In retrospect, I’d you can get the connections to work, HKG could have been better, as you can travel onwards on Cathay for points as well. AirAsia is pretty cheap though but flies from the ‘wrong’ Bangkok airport for connecting from Europe.