Problems with vacuum

stat_activity.current_query,pg_stat_activity.query_start FROM pg_stat_activity WHERE pg_stat_activity.current_query ~ 'public.mensagem' AND pg_stat_activity.procpid!=pg_backend_pid();
-[ RECORD 1 ]-+-----------------------------------------------------------
procpid       | 2738
current_query | autovacuum: VACUUM public.mensagem (to prevent wraparound)
query_start   | 2013-08-27 18:58:41.527238-03

I tried to cancel it but unsuccessful.

billing=# SELECT pg_cancel_backend(2738);
-[ RECORD 1 ]-----+--
pg_cancel_backend | t

billing=# SELECT localtimestamp(0);
-[ RECORD 1 ]------------------
timestamp | 2013-09-04 11:23:18

billing=# SELECT pg_stat_activity.procpid,pg_stat_activity.current_query,pg_stat_activity.query_start FROM pg_stat_activity WHERE pg_stat_activity.current_query ~ 'public.mensagem' AND pg_stat_activity.procpid!=pg_backend_pid();
-[ RECORD 1 ]-+-----------------------------------------------------------
procpid       | 2738
current_query | autovacuum: VACUUM public.mensagem (to prevent wraparound)
query_start   | 2013-08-27 18:58:41.527238-03

Any idea? Any tips?

Best regards