I bet you have one, a fire ant problem (disc mowers and fie ant hills don't get along well). If you are running high lift knives, I suggest going to a flatter knife (they come in a couple configurations). I get mine from Shoup. USA made and Shoup offers 2 different angles. I use the high lift knives here but I don't have a fire ant hill issue either. Shoup is about 1/2 the price for knives that your dealer charges btw.
Not familiar with Vermeer, there aren't any around here, only JD and NH and a smattering of Kubota (Vicon) mowers. I have a NH and my belt is under constant tension, I have a spring loaded idler (much like the idler on an automotive serpentine belt). My drive belt is a siamesed 5 section V belt with a one piece top cover. I've never replaced it not have I had any issues with my drive sheave from the right angle gearbox. My main drive pulley has a collet type lock on it with 3 bolts and they are safety wired so they don't loosen up.
If it's that new it should be under warranty. Did you inquire from your dealer as to if they would provide you with a loaner machine?