Do I need a heavy winter jacket for Northern Vietnam?

Only if you are traveling in January. Otherwise, a light fleece or hoodie is sufficient for mountain regions or air-conditioned buses.

Is it better to bring a suitcase or a backpack?

Backpack. Streets are often unpaved or narrow, and navigating transit hubs with rolling luggage is difficult.

Packing for a Southeast Asia Backpacking Trip

Pack for a hot, humid climate using breathable synthetic fabrics and keep your total bag weight under 22 pounds (10 kg). Focus on modular clothing and versatile gear that allows you to carry your entire setup as a carry-on to avoid checked bag fees and lost luggage.

Select the right bag. Use a 40L travel backpack that opens like a suitcase (clamshell) rather than a top-loader. This allows you to see and access your gear without dumping your bag on the floor.
Adopt a 'one-week' wardrobe rule. Bring enough clothes for 7 days regardless of trip length. You will find cheap laundry services everywhere in SEA that charge by the kilogram (usually $1–$2 per kilo). Don't pack for a month; pack for a week.
Prioritize fabric over style. Avoid heavy denim or thick cotton. Pack merino wool or lightweight synthetic moisture-wicking shirts and shorts that dry overnight. You will be sweating; these fabrics prevent chafing and smell.
Use packing cubes. Use three medium-sized cubes: one for tops, one for bottoms/undergarments, and one for dirty laundry. This keeps your bag organized and compressed.

Do I need a heavy winter jacket for Northern Vietnam?: Only if you are traveling in January. Otherwise, a light fleece or hoodie is sufficient for mountain regions or air-conditioned buses.
Is it better to bring a suitcase or a backpack?: Backpack. Streets are often unpaved or narrow, and navigating transit hubs with rolling luggage is difficult.