Robbed in Hanoi

Yesterday around 11 am I was almost robbed in front of my hotel in Old Quarter. I just got off a Grab Motor and paid in cash, the rider saw my money, but also that thief probably from a distance. Then I put my money back in my shoulder bag and walked to my hotel. At that moment a 35-40 year old Vietnamese man came up to me and opened some posters on top of my shoulder bag and dug through my bag. I then grabbed his hands really hard and took my money back. I hesitated for a long time about giving him a few punches, but I didn't want to get into trouble later. He ran away when I let him go, but in the future what is the law surrounding this? If someone robs you, you can hit them and how do the Vietnamese police deal with this?