I have conditioned all of my veg tanned MGs and it hasn't changed and of them by much at all. Once it dries, it's pretty much back to normal. It's definitely not a drastic change. It does help soften the appearance of any scratches. I've used Apple Conditioner so far. I also have a conditioner I got in Paris at MM7, the official Louboutin cobbler. I haven't tried that one yet, but I'm not nervous about it.
I haven't used an actual protectant yet, since I'm in LA, and it doesn't rain much here. The conditioner provides light protection. But, again, I wouldn't be too nervous. These bags aren't as fragile as one might think.