Hanoi is flat, but Vietnam is not. Ride toward Ba Vi, Tam Dao or any river valley and you will stop on a slope at least once — often at a junction with a truck behind you. A hill start is a technique problem, and every transmission type has its own answer.
Manual bikes: front brake holds you
Keep the front brake on with two fingers as you release the clutch until you feel the engine start to pull, then release the brake and add throttle. The bite point does the work — your goal is zero roll-back, not speed.
Scooters: brake, then throttle
Automatic scooters make this easy but not automatic. Hold the rear (or front) brake, twist the throttle until you feel the transmission push, release the brake and go. The trap is rushing: a jerky release can launch you into the traffic ahead. Practise once on a quiet incline.
Semi-automatics: the middle case
On a semi-automatic, hold the rear brake, give steady throttle, and let the clutch engage as you release. These bikes start gently, so a little more throttle than feels natural is usually right.
On gravel or wet slopes
Any slope with loose surface or wet paint doubles the stakes: roll-back can turn into a slide. Pull away even more gently, keep the front straight, and if the hill is steep and busy, it is legitimate to wait for a bigger gap rather than starting under pressure.
Practise deliberately
Find a quiet incline — a bridge approach early on Sunday morning works — and do five starts with each hand. Check that the brakes feel strong as part of your motorbike-safety-checklist-before-riding routine, because a hill start with weak brakes is where rental wear shows first.