My VPI Classic 3 belt does the same thing. I powdered it as recommended by VPI but as you said, it doesn't last long. I finally got used to the sound which stops after the platter gets up to speed and use it that way since it doesn't affect the sound on the record. I still love the TT.
Classic 3 VPI
My Classic 3"s belt squeals upon turn on. After a couple of seconds, the squealing stops. I've tightened the pulley, installed a new belt and did what VPI recommended, I powdered the belt. After applying powder, the squeal quietened but squealed again on the very next try. I reapplied powder and the same thing happened. Any advice is welcomed.