Personally, I would setup cases like this with an orientation toward your users. I would make the case an action you need to take instead of one the vendor needs to. For example, if you are waiting for a part, instead of the case "Deliver part" assigned to a virtual user "Vendor", I would title the case "Ensure part delivery" and assign it to the person in your company responsible for either managing the vendor or performing the next task which depends on the vendor. This way all of your cases are accountable within your group of users for tracking.
If you prefer, I think a system of cases assigned to vendor virtual users is a good idea. You can periodically view filters of cases which are due and assigned to them in order to follow up, and when you list all cases in a milestone, you will see them there as well.
Since you won't be performing actual work for the case (the vendor is), I wouldn't put an estimate on it for EBS. Instead, you can set a start date dependency for a milestone containing all work that depends on that vendor if it is truly blocking.