vacuumdb -v output

Hi,
I've just put a system into production in which some tables are updated frequently - several times per second.
I'm doing a nightly vacuumdb -v, but am not sure if it's achieving anything. Here's the output for one table:

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INFO: vacuuming "public.fip_track_circuit"
INFO: index "fip_track_circuit_pk" now contains 1557427 row versions in 4538 pages
DETAIL: 10 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.22s/0.14u sec elapsed 6.51 sec.
INFO: "fip_track_circuit": removed 10 row versions in 9 pages
DETAIL: CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "fip_track_circuit": found 10 removable, 1557427 nonremovable row versions in 14305 pages
DETAIL: 1555321 dead row versions cannot be removed yet.
There were 1 unused item pointers.
0 pages are entirely empty.
CPU 0.42s/0.24u sec elapsed 6.82 sec.
INFO: vacuuming "public.fip_xl_switch"
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I'm concerned about ".... 1555321 dead row versions cannot be removed yet...."

I'd be extremely grateful if anyone could cast some light on this.
Thanks, Kevin