With seven days I would definitely stick to one country. Certainly there's plenty to do in Chiang Mai and the surrounding countryside to fill a week. I can't speak to Vietnam as I'm also planning my first trip there.

With seven days, I would also consider a week in Penang/Langkawi or maybe even heading down to Sarawak for Kuching and the nearby national parks. If you don't mind some long bus rides, the trio of Kanchanaburi, Ayutthaya and Sukhothai in central Thailand might be interesting and would keep you away from the big cities, though seven days might be too tight. Train from Bangkok to Kanchanaburi, bus to Ayutthaya, train/bus to Sukhothai, fly back via Bangkok. Plenty of history and culture in those three though.

Regarding points, flights from HKG to SE Asia are often cheap—you might just want to pay cash. I'm assuming you already booked a flight to HKG (or are there now?) and just need to book from HKG to your destination? Do you have any Avios (or means to get them—Chase UR, Amex MR, etc)? HKG-CNX RT is 15k Avios and CX/KA tend to have good award availability at short notice, plus their ex-HKG network to SE Asia is far more extensive than *A.