The Need to Vacuum
As Redshift does not reclaim free space automatically, updates and delete operations can frequently lead to table growth. Equally, it’s important as new entries are added, that the data is maintained in a sorted sequence.
The VACUUM command is used to re-sequence data, and reclaim disk space as a result of DELETE and UPDATE operations. Although it won’t block other processes, it can be a resource-intensive operation, especially for data stored using interleaved sort keys.
It should be run periodically to ensure consistent performance and to reduce disk usage.
Some of this is good Postgres advice; some of it is good MPP advice (and serves well, for example, when dealing with Azure SQL Data Warehouse); the rest is Redshift-specific.