VIM version 8.0.1187 (and other versions most likely) ignores umask when creating a swap file ("[ORIGINAL_FILENAME].swp") resulting in files that may be world readable or otherwise accessible in ways not intended by the user running the vi binary.
http://www.openwall.com/lists/oss-security/2017/10/31/1
http://security.cucumberlinux.com/security/details.php?id=120