This is quite an ambitious plan to visit three countries in just 2.5 weeks. There are many places to visit in each country. I can give some suggestions for Da Nang and HoChiMinh city trips in Vietnam. I am not sure about cooking class. However, guesthouse and small hotels can be searched easily in the internet. In HoChiMinh city, tourists mostly stay in Pham Ngu Lao street. HoChiMinh city is quite a busy one and there are a lot of things to do and food to eat in the city.
DaNang is a small but very nice city. If only for tourist purpose, that person can choose a guesthouse or small hotel in DaNang city, take a tour to the Ancient Hoi An town (just 30 km away) by taxi. If it happens to be the middle of the lunar month, the night in Hoi An would be very romantic as latens will be lighted up along the streets and the river.
Travelling between HoChiMinh city and PP can be through air or road. If you choose the road traveling, which many tourists would like to, you should contact the travelling agent to enquire about visa at the border gate.
The weather at these months towards the end of the year is hot and rainny in all these cities. But the North of Vietnam is starting its very cold winter (as cold as Melbourne).
You can take Australia dollar to Vietnam and get changed easily. However, US dollar is still more common in all three countries. Take 50$ or 100$ notes for easier to get changed.