so you can either model it as:

definition vendor {
  relation custom: vendor
  permission edit = tenant->admin + (owner & custom)
}

and write a relation on the vendor with the same ID at both ends, or you can model it as a wildcard relation:

definition vendor {
  relation custom: vendor:*  // could also be user:*; the specific relation doesn't matter a ton
  permission edit = tenant->admin + (owner & custom)
}

and write a relation of

vendor:*

on the vendor