Is postgres vacuum secure
Citing the reference:


  VACUUM FULL rewrites the entire contents of the table into a new disk
  file with no extra space, allowing unused space to be returned to the
  operating system


Without FULL, the database files are not even truncated, so data may still be in the files. With FULL, deleted data should be gone from the files (since they are recreated). However, data may still be in deleted parts of the underlying file system of course.

In any case, if it is important to you that data is destroyed, you must probably erase or destroy the disk.