Does amazon rds PostgreSQL require vacuum? It isn't clear if auto vacuum is running in rds. Thanks!
Run this query to show if/when your tables were last vacuumed.

SELECT * FROM pg_stat_all_tables;

A table won't be vacuumed until the insert/update/delete threshold is reached, which is 20%.

You can tweak the setting in RDS by creating a new DB Parameter Group and change the setting you want.