Free users don't care if you tell them to fuck off.
The company paying $10k can't change vendor overnight so it's in their best interest to stay in good terms. Also in his company, the guy you're talking to don't want to be "the guy who pissed off that key vendor".
I second that. Once a vendor becomes important enough I even stop thinking of it as a vendor and rather as a partner that you interact with as if it was part of your own organisation. I am not amused when members of my team are rude to a vendor and jeopardise a relationship that took a long time to build. Even when you pay six figures a month, you can still get preferential treatment compared to other customers in the same tier when it comes to influence over the roadmap if you actively foster the relationship.
> Free users don't care if you tell them to fuck off.
You don't have to and shouldn't respond in any specific time period. Even if you're the sole maintainer of a critical project and you imagine the world wants your head, the world has to wait sometimes.
However, users may abandon projects where support tickets go untended, maybe even writing a post about it in the process[1], so try to respond when you can, unless you've abandoned the project.
As a representative for a FOSS project or even a bystander commenting on a PR, respond professionally and succinctly.
If you shouldn't accept a PR, don't.
If it's a request that seems hostile to the project, and you have time, either leave it for a little while to cool off, try to serve it with professionalism without getting off-topic, or at worst close it.
If needed, add a "code of conduct"[2] so that you can "encourage a pleasant and productive environment by responding to disruptive behavior in a fast, fair way"[3]. Note that instituting one before it's needed, while seeming proactive, may put off some users.
If you screw-up, apologize briefly, then fix it or move on.
The company paying $10k can't change vendor overnight so it's in their best interest to stay in good terms. Also in his company, the guy you're talking to don't want to be "the guy who pissed off that key vendor".