I'm traveling to Vietnam and was wondering how much I should be on gaurd about theives looking to cut my camera strap and run off or grab it while driving by?
It's a nice looking SLR camera with a big lens.
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How often do cameras get stolen? Every. Single. Day.
You should always guard your camera, and any other valuables, as if they were expensive, and could walk off at any time because, well, they are expensive, and could walk off at any time.
Certainly the degree to which you guard your belongings can be very region-dependent, but will generally be a lot more localized than "SE Asia." It would be more like one neighborhood versus another. Or one parking lot versus another.
There are some things you can do to reduce your risk:
Travel with a cheaper camera.
Unless you're a professional photographer on assignment, there's probably no reason to carry an SLR. Consider a smaller camera, which is both easier to hide, and less of an attractive target for theft. It also much less annoying to travel with less stuff.
If you really must travel with an SLR, travel with the cheapest, smallest, and least-attractive SLR and lens possible to get your job done.
Keep your camera well hidden, except when you are using it.
Carry it in a bag that doesn't look like a camera bag. Don't carry it at all when you aren't going to be using it--leave it locked in the hotel safe, or some other secure location.
Cover up any logos.
Use black electric tape or some other mechanism to cover the logos on your cameras. This makes them look "generic" to a casual observer, and less of a target. And don't use the standard Nikon- or Canon-branded strap that came with your camera for the same reason. You can buy an unbranded camera strap very cheaply, or better, consider a strap upgrade, and one with a steel cable...
Consider a camera strap with a steel-cable.
I use the California Sun Sniper Steel-Steel Reinforced Camera Strap. Aside from being far more convenient and usable than the stock over-the-neck camera straps, it has an embedded steel cable, to prevent a thief from grabbing your camera, cutting the cable with a knife, and running away.
Familiarize yourself with common thieving techniques.
A book I highly recommend is Travel Advisory: How to Avoid Thefts, Cons, and Street Scams. It covers a lot more than just cameras, and helps you guard yourself against all sorts of scams and thievery.