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To recruit one of the top-level vendors, at least one of your workshops must fulfill the following requirements:
(1) A matching vendor stand must already exist at the workshop.Thus, iIf you want to recruit Rylee, there must be a MiscItem vendor stand existing. This vendor stand must have the highest level, but whether it is unassigned or already assigned to somebody else doesn't matter.
(2) The total linked population rating of that workshop must be larger than a minimum value
The total linked population rating of a workshop is equal to that workshop's population plus the sum of the populations of all workshops that are connected to it by supply lines.
The minimum values vary from 10 to 40, depending on the trader type (they are defined in properties of WorkshopParentScript)

Both requirements must be fulfilled at the same workshop. This is because the scripts will not start evaluating criterion (2) if criterion(1) is not fulfilled (this also saves the script the work to check the individual perks required for hiring a top-level vendor: if you don't have them, you can't build the required vendor stand ). Once at least one workshop fulfills both criterions, the dialogue option will eventually become available.