I'd +1 for triggered_by over just trigger, but I actually think triggered_by should be avoided as well for the same reason (term overload). We already use the verb "launch" as the word to describe this action in the existing autovacuum docs, but I was unable to come up with something that both sounded good and made sense for the manual vacuum case, so now I would lean towards "initialized_by".
What about “started_by” ? it’s unambiguous and consistent with other columns
