You can find some authorized SYM shops in Hanoi in the link below.
My husband & I some times had our SYM motobikes maintained and spare parts changed at 8 Yet Kieu street (near Hanoi railway station), tel. (04) 38221881. They have trustful factory-made spare parts and experience mechanicians. They can come to your place to get your bike to the shop if you pay for the fees. As our experience, we always try to do this way...
- Take you bike to the shop which is near your house and get it pumped.
- Drive the bike to the shop and ask them to check its tyre to make sure that they can fix it instead of changing a new one. (you don't have to pay much while your tyre is actually still in good condition).
Inbox me if you need further instruction :) or want me to call and talk to them.