Backpacking trip

First solo Southeast Asia trip (Jan–Feb) — Vietnam + Thailand recommendations?
I’m 19 from the UK and planning my first big solo trip around Jan–Feb next year.
The plan is to start in Vietnam, flying into Hanoi and travelling north to south. I want to do the Ha Giang Loop, explore the north, then slowly make my way down towards central Vietnam. I’m currently thinking of flying from Da Nang to Thailand afterwards and spending around 10 days there before heading home.
I’m thinking:
2–3 days in Bangkok
then heading somewhere else for the rest of my time
The main thing I’m looking for in Thailand is a good social/backpacker scene. I’ll be travelling solo, so I want somewhere where it’s easy to meet other young travellers (around my age), with good nightlife. I’m more into bars, meeting people, live music, beach bars etc. rather than only big rave/club scenes, but I’m open to trying anything.
I’ve looked at places like Koh Phangan, Koh Tao, Pai, Chiang Mai, Phuket, Krabi etc. but there are so many different opinions online that I’m struggling to decide what fits best.
A few questions:
Where would you recommend for a solo 19-year-old looking to meet other backpackers and have a fun final week?
Is Jan–Feb a good time for Vietnam + Thailand in terms of weather and backpacker numbers?
Would you spend the full 10 days in one place, or split it between a couple of places?
Any advice from people who have done something similar would be appreciated!