Please note that this is how it should work by default for both vendors and listings. The category can be selected only when added on the frontend and cannot be changed afterward, as the form itself may change, and it will already be a different type of listing (since different packages, attributes, etc. can be attached to the category). However, note that administrators can always change the category.
Similar to vendors, it is possible to specify a category only when filling out the Complete Profile when registering a vendor. After registration, it is not possible to change it, as the profile fields may depend on it.