It really depends on how you want to build your app. Are you going to spin up a new instance for each vendor, or are you going to have one large multi-tenant instance? Will customers be able to order from multiple vendors in one order or will they need to order from each vendor individually?
I don’t have much experience with large, heavy-use sites. But at a base level I’d deploy Docker instances through Kamal. I’d read the guides as they are pretty helpful.
Also, look at some large, heavy-use open source projects to see how they do it:
Redmine Probably not super busy site, but it’s got a lot of functionality.
I’m sure there’s other large RoR OSS out there; I just can’t think of any at the moment.
Cheers
Shannon