vacuumdb hanging database cluster

When I run:
vacuumdb --full --all --analyze --quiet
on my database cluster it will complete in < 2 minutes (this cluster
is a few million total rows and ~2GB).

After testing, I set this up as an off-hours cron job and it worked
fine for several days then hung the whole database. After my pager
pulled me from bed I found the vacuumdb process still running but the
vacuum process on the first database (alphabetically) was showing it
was waiting:
postgres: vacuumdb --full --all --analyze --quiet
postgres: postgres firstdb [local] VACUUM waiting

A couple hundred processes were showing as "startup waiting" and one
was "idle in transaction". The process in the "VACUUM waiting" state
was the only one connected to that database - all other connections
were to other databases.

CPU and disk utilization were essentially zero. Suspecting a lock
problem I attempted to use a pre-existing connection to view pg_locks
but it would not respond.

I killed the vacuum process and all the processes in the "waiting"
states cleared within a second or two and system returned to normal.
The pg_locks query also returned but showed no useful info.

I tracked down the process that was "idle in transaction" and it was a
pg_dump process running on another machine. This process does a
periodic dump of one very small table and should complete in a
fraction of a second but was still waiting since the previous day -
apparently without deleterious effects.

There was no useful info in the log.

I've stopped running the vacuum full job via cron till I can trust it.
Any ideas on how to track/prevent this behavior? Server is version
7.4.1 and my web searches have proved futile.

Cheers,
Steve