That's one of the things many local MN shooters have been dealing with in the past year or two... With Defensive Edge training/consulting right in our own back yard we're able to take LOTS of dynamic training courses... Last weekend I was at a Tactical Carbine course as well.
The thing is... When there's that high of a volume of fire, there can be a VERY increased chance of injury/death. Not so much due to being directly shot (though it is a possibility), the largest threat I've seen is RICOCHETs. I have been hit in the Abdomen with a ricochet (at a machinegun shoot), but it luckily didn't break the skin (but it left a welt like a paintball hit). Another AR15.com member took a ricochet that pierced his shoulder and knocked him down pretty hard. A good vest quite possibly would have stopped that.
First, only shoot at a SAFE, WELL MAINTAINED shooting range with a clean backstop devoid of rocks, steel garbage, or other hard surfaces that may cause ricochets. I was never injured at a dynamic training course/event, nor was anyone else that I know... However, where there is high volumes of fire (potentially at a training event, and certainly at MG Shoots), body armor is a VERY good idea, IMHO.
I'm soon purchasing a level II or IIA kevlar vest, and later will buy an Armor Plate to put into my tactical chest-gear/carrier setup that I currently use in courses.
I suggest getting soft body armor for the time being, and later you can decide if armor plates are also necessary to supplement your situation...
You need to first assess your needs, and purchase gear/equipment in order of necessity.
For example:
I need body armor, but I also need to finish my vest/carrier assembly the way I want first...
I'll spend $75 more on gear & pouches...
I will buy all new 30rd mags to replace the older pre-94 mags I've been using for umpteen years... ($120)
THEN I'll buy my soft Kevlar armor (mostly for ricochets).
Maybe later I'll buy a plate for direct rifle shots...
So after ~$350-$400 I'll have finished my setup to my satisfaction (before plates).
Just prioritize the purchases (unless you're rich, but that's a problem I don't have the luxury of suffering from).
Basically...Definitely get the soft armor, and add a plate carrier over the top if you need it in the future.
-inuhbad