VACUUM doesn't run in parallel on a single table.
If you want to run multiple VACUUMs at once you can do it like this, for example, running four vacuums at once to scan all databases:
$ vacuumdb --jobs=4 --all
If you run multiple VACUUMs at once you'll use more memory and I/O, so be careful. The exact calculation is complex, especially if you have tables with custom VACUUM settings, but the general idea is that I/O can be slowed down by raising vacuum_cost_delay or lowering vacuum_cost_limit.