Vacuum doesn't end

I have an application that may add a couple million rows per day so I
vacuum nightly.  The tables never get to more than about 10 million rows
before I move off the interesting information to other media.  Somewhat
often, my vacuums don't complete as shown from this ps command (usually
VACUUMs take a couple minutes)

4442 ?        R    949:07 postgres: postgres tag 127.0.0.1(33420) VACUUM

and the CPU for postmaster (hyperthreaded linux) will be in the high
90s.

The locks look like:
scouts=# select * from pg_locks;
 relation | database | transaction |  pid  |           mode           |
granted
----------+----------+-------------+-------+--------------------------+---------
          |          |    12826125 | 11642 | ExclusiveLock            |
t
    16839 |    17230 |             | 11642 | AccessShareLock          |
t
    17251 |    17230 |             |  4442 | RowExclusiveLock         |
t
    17251 |    17230 |             |  4442 | ShareUpdateExclusiveLock |
t
    17246 |    17230 |             |  4442 | ShareUpdateExclusiveLock |
t
    17246 |    17230 |             |  4442 | ShareUpdateExclusiveLock |
t
          |          |    12817402 |  4442 | ExclusiveLock            |
t
(7 rows)


I all inserts and maintenance through JDBC and may have inserts going on
while a different java thread calls the VACUUM command.

Any thoughts?  Can I recover without dropping the server?

Thanks for any help,
Scott