Is carrying DSLR safe Vietnam?

I suggest you keep your camera and equipment well out of sight until you wish to shoot, take out your camera, shoot, then return the camera to your bag. Walking around with a camera  hanging around your neck is an invitation for some one to rip it from you.

Also, ALWAYS keep one hand on your bag, which you should carry across your chest and not hanging off one shoulder, so that you can prevent a thief from ripping it off  you.  When two thieves on a motor bike tried to steal my shoulder bag, all they got was the strap and I still was firmly holding my bag against my stomach.

Good thinking, but you have already answered your own question: self-awareness.   And you show that.

Common sense dictates you don't carry visible signs around.   Use in crowds?  Self-explanatory too.   So even armed guards cannot prevent the omnipresent threat of theft.   In fact, they draw attention...

One option is to carry within a larger bag securely attached to you.  The equally obvious danger is being dragged away as well.    It's happened before, and unhappily for the owner...
 
Be aware - and aware..?   You can never cover all the bases.   Cover with a soft cloth such as a dark scarf which will often even hide the fact you are taking photos.   Do NOT use this trick in China...

Empathy?   Sure.  Think as a thief would.   Without becoming one.      Too obvious?  Too rarely...

..but I think I'm probably preaching to the choir....