So why exactly do we have to eat so slow with a VSG

You've gotten great replies here....
You eat too fast and you don't realize your stomach is FULL already and those last 2 bites will have you vomiting that food up because there's no room plus it'll hurt!
Take your time eating with each bite. Pay attention to how you feel.  Swallow your food and wait a minute to swallow again because sometimes the fullness sneaks up and after you are thinking "I shouldn't have taken those last 2 bites" and it feels like you need to burp something up but food will come up or you'll have chest pains.

Hi Baritric Babe! 

I'm more like Ros-Mari, I don't eat slowly anymore and I don't bother chewing anything to death.  I eat normally, and I'm done.  I also found the same thing as she did - that if I eat slowly and chew everything til its disgusting mush I can actually eat more.  And it's VERY unpleasant!  Food is just not that interesting that I want to be sitting in front of it for that long, lol

I also never eat until I feel full.  I just eat a few bites and stop and then feel full a bit later.  
I have eaten too much before and got the foamies like Old Medic described.  It's awful!!  I haven't done it in months now though, and I don't think I will again, knock wood!