I found a decent full-faced helmet towards the south end of Pho Hue, near Dai Co Viet street. If you're looking for the expensive ($100+) brands found back home, I'm not sure where to go. I'm guessing that the Vespa shops or other places selling new motorbikes can help you.

The full-faced helmet I bought took quite a knock when my ex-gf crashed our bike. I smacked my head on the pavement pretty hard but bounced right up like nothing had happened. It cracked the helmet, but it did its job. Not bad for 200,000 dong.

I just bought a new helmet at Yamaha Town near the Vincom Towers.
They have full faced helmets, but I went with one that was not full faced. I'm not ready to make that move yet. The one I did by covers the cheeks and back of the neck. I've had it for a few weeks and I love it. There is space and cushion of the ears. It also has a face shield that will be great for the cold winter that is sure to come as well as in the rainy season.
The cost was 800,000 vnd. A lot more then just around the corner on Pho Hue, but I like knowing I have the real thing.
The helmets are made in Thailand.
Good luck and drive safe!

Oh, and I ended up getting one for me and one for him!

An 800,000VND helmet can not be much, when I certified helmet starts at $200 for the most basic model.

I am able to source helmets from Singapore. There are a variety of sizes, colours and styles.

The prices range from $200SGD to over $1000 SGD.

They can be viewed online, then a few measurements and the corect size can be ordered.

These helmets are SNELL and DOT certified.

Shipping to Vietnam will cost about $35USD, and takes about 2 or 3 days.

PM me for details

some international organizations get helmets from this shop near kim ma. No.8 Ngoc Khanh street, tel: 7710671