And Just to reiterate my own understanding of this...
autovacuum priority is less than a user-initiated request, so issuing a manual vacuum (user-initiated request) will not result in being cancelled.
Somethings happen in some situations and not in others. I don't know that it is useful to categorize them into a monotonic priority scale.
Autovacs "to prevent wraparound" don't get cancelled the way ordinary autovacs do, but they still use autovac IO throttling settings, not the unthrottled (by default settings) manual vacuum settings, which can be a major problem sometimes.
Note that no kind of vacuum should normally get cancelled using the signalling mechanism during truncation phase, that seems to be due to some rather extreme situation with IO congestion.
