How do I repair a vendor relationship?

I'm looking for some advice regarding mending a vendor relationship. I have a vendor with whom we've done business for many years, and the relationship has been deteriorating over the last couple years. This vendor has grown considerably, and we are no longer his largest customer, yet we rely on this vendor for the bulk of our molding work. If it gets to the point that we are told to come get our molds and find a new vendor, we would be in a terrible predicament. Help!