Yes, the VACUUM table_x RETAIN 720 HOURS;  command will indeed override your table-level retention properties and potentially compromise your 6-month time travel capability. 
When you explicitly specify a retention period in the VACUUM command, it takes precedence over the table properties 'delta.deletedFileRetentionDuration'  and 'delta.logRetentionDuration' .

In your case, the maintenance job is enforcing a 30-day retention policy that conflicts with your 180-day table configuration. The vacuum operation will remove data files older than 30 days that may still be referenced by transaction logs within your 180-day retention window.
Hope this helps!