Invulnerable VACUUM process thrashing everything

A few WEEKS ago, the autovacuum on my instance of pg 7.4 unilaterally
decided to VACUUM a table which has not been updated in over a year and
is more than one terabyte on the disk. Because of the very high
transaction load on this database, this VACUUM has been ruining
performance for almost a month. Unfortunately is seems invulnerable to
killing by signals:

# ps ax | grep VACUUM
15308 ? D 588:00 postgres: postgres skunk [local] VACUUM
# kill -HUP 15308
# ps ax | grep VACUUM
15308 ? D 588:00 postgres: postgres skunk [local] VACUUM
# kill -INT 15308
# ps ax | grep VACUUM
15308 ? D 588:00 postgres: postgres skunk [local] VACUUM
# kill -PIPE 15308
# ps ax | grep VACUUM
15308 ? D 588:00 postgres: postgres skunk [local] VACUUM

o/~ But the cat came back, the very next day ...

I assume that if I kill this with SIGKILL, that will bring down every
other postgres process, so that should be avoided. But surely there is
a way to interrupt this. If I had some reason to shut down the
instance, I'd be screwed, it seems.

-jwb