Postgres manual vacuum and auto vacuum running on the same table

I don't think that you can tell from the "verbose" listing whether any pages were skipped because they were known to contain only visible tuples.

I think that it will only vacuum pages flagged as "all visible" if it decides to freeze tuples in the table, either because of approaching transaction wraparound or because you explicitly ran VACUUM with the FREEZE option. (If you use the vacuumdb command-line executable, this is the -F option.)

In our shop we supplement the autovacuum activity with a scheduled VACUUM FREEZE ANALYZE on the database during non-peak hours. That minimizes the overhead of autovacuum during peak hours, and slightly improves query performance, because visibility checks are simpler.

1) If you don't count your own time as a resource, then you should always be able to hand-craft a vacuum schedule which uses fewer total resources than autovacuum does. If you do count your own time, this is almost surely not worthwhile.

2) Other than manually or algorithmically turning it on or off, no. Nor would it make sense to do that. A database that sees only a very steady predictable load with no unusual spikes still needs vacuuming. However, if you do spend a lot of time coming up with a highly optimized hand-crafted vacuum schedule, then autovacuum will never have any vacuuming to do except under unusual activity not anticipated in your custom schedule; so in this sense what you want is automatically the case.

3) It isn't clear whether you mean when an autovacuum worker will start up in a given database, or when any given table will start to be vacuumed. The first is determined by autovacuum_naptime, unless autovacuum_max_workers becomes the limiting factor. The second is determined by when the table crosses the threshold, with a granularity of autovacuum_naptime, except when autovacuum_max_workers becomes the limiting factor. How accurately you can predict this depends on how steady the activity on that table is, and whether the vacuum tasks are generally overloaded or are mostly idle.