One thing to keep in mind is that you’re not the only customer of the vendor. You might have a specific representative from that vendor as your go-to, but the vendor may have other orders for products, maintenance of machines, etc. They may have to balance orders/scheduling with dozens of other companies, not just yours. The most basic way to keep a vendor happy is to treat the representatives/company as you want to if you were in their shoes. Are you rough and overly demanding, or are you pleasant and professional? Being clear about expectations (contracts/agreements), proper planning, mutual respect, and the rest that has been said above are all key parts of this basic rule.