Motorcycle gear in Thailand and Vietnam

Hi.

I'm planning to go to SEA for a couple of months. I've got full motorcycle license in Poland a couple of months ago in preparations to rent or buy a motorbike (prob ~250-500cc) either in Vietnam or Thailand.

I have a couple of questions though:

1. Considering I'm going from Poland and already have some summer mesh/textile gear would you think it's better to take it with me in checked luggage or buy new moto gear there?

2. Are those water cooling vests any good in hot and humid countries? Could you recommend which one?

3. My original plan was to go to Hanoi (Vietnam) and buy a bike there then travel south. Then drive to Cambodia and finally Thailand. But from what I've heard it's not feasible to buy a bike in Vietnam and travel to Thailand (and vice versa) as you need some paperwork and it's too much hassle?

4. If I buy a bike in Vietnam then fly to Thailand for 2 months could I leave the motorbike in Vietnam and come back to it later? Or will it get immediately stolen? Should I just sell it when leaving then?

5. I never drove a motorbike other then on the course (just the bare min 20h on MT-07). Will I be fine xD

6. I don't know if this is the right place to ask, how do you cross post to different subreddits? Maybe r/ThailandTourism would be better to ask?