It is well advised to siwtch off autovacuum for a few tables when using Slony 1.2.x.

Have a look at http://slony.info/documentation/1.2/maintenance.html (Section 6.1 Interaction with PostgreSQL autovaccuum).

Here are the SQL query
-- Slony tables
-- NOTICE: IN THE LINE BELOW, REPLACE clustername by the real cluster name.
INSERT INTO pg_catalog.pg_autovacuum 
    (vacrelid, enabled, vac_base_thresh, vac_scale_factor, anl_base_thresh, anl_scale_factor, vac_cost_delay, vac_cost_limit, freeze_min_age, freeze_max_age)
        SELECT oid, 'f', -1, -1, -1, -1, -1, -1, -1, -1 FROM pg_catalog.pg_class WHERErelnamespace = (SELECT OID FROM pg_namespace WHERE nspname = '_' || 'clustername') AND relhasindex;

-- System tables				
INSERT INTO pg_catalog.pg_autovacuum 
    (vacrelid, enabled, vac_base_thresh, vac_scale_factor, anl_base_thresh, anl_scale_factor, vac_cost_delay, vac_cost_limit, freeze_min_age, freeze_max_age) 
		SELECT oid, 'f', -1, -1, -1, -1, -1, -1, -1, -1 FROM pg_catalog.pg_class WHERE relname in ('pg_listener','pg_statistic') and relnamespace=(select oid from pg_namespace where nspname='pg_catalog') ;