When Vendors don't answer calls, emails, voicemails?

Although I was never involved in an online, or nation wide business, I did spend almost 50 years in a retail business. What I preached to my employees, I believe still applies.
Anytime you give your customer a reason to go to your competition, you give your competition an opportunity to take your customer.

Let's be honest here, there is plenty of customers you WANT your competition to take off your hands. While the customer is usually right, sometimes the customer is an A hole.

I believe myself to have good customer service, too good sometimes maybe. But I get very impatient with "time vampires" or people who clearly cannot be made happy. Once they light one end of the bridge on fire I go ahead and light the other lol.