VMP are using folks like you for short-term financing. They should be paying you a fee or interest.
My wife tells me some suppliers do this to small businesses. Make an 'error' on their orders (overshipping/overbilling) or just making billing 'mistakes,' then taking a while to refund the overcharges. It's how they create 'float' or free short-term loans on someone else's dime.
Shady AF.