Should dovecot not be using different severity levels like auth.warn? On my system everything goes to loglevel info:

lev_info:Aug 9 16:18:24 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session= lev_info:Aug 9 16:18:29 mail03 dovecot: auth-worker(28656): pam(krinfo,188.206.104.240,): unknown user lev_info:Aug 9 16:18:50 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 25 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS: Disconnected, session= lev_info:Aug 9 16:18:53 mail03 dovecot: auth-worker(28656): pam(krinfo,188.206.104.240,): unknown user lev_info:Aug 9 16:19:01 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 8 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session= lev_info:Aug 9 16:19:13 mail03 dovecot: auth-worker(28656): pam(krinfo,188.206.104.240,): unknown user lev_info:Aug 9 16:19:15 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session= lev_info:Aug 9 16:19:24 mail03 dovecot: auth-worker(28656): pam(krinfo,188.206.104.240,): unknown user lev_info:Aug 9 16:19:26 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session= lev_info:Aug 9 16:19:27 mail03 dovecot: auth-worker(28656): pam(krinfo,188.206.104.240,): unknown user lev_info:Aug 9 16:19:29 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session= lev_info:Aug 9 16:19:47 mail03 dovecot: auth-worker(29664): pam(krinfo,188.206.104.240,<14Pb3a+Pih68zmjw>): unknown user lev_info:Aug 9 16:19:49 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session=<14Pb3a+Pih68zmjw> lev_info:Aug 9 16:19:51 mail03 dovecot: auth-worker(29664): pam(krinfo,188.206.104.240,<99cO3q+Pix68zmjw>): unknown user lev_info:Aug 9 16:19:53 mail03 dovecot: imap-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<xxxxx>, method=PLAIN, rip=x.x.x.x, lip=x.x.x.x, TLS, session=<99cO3q+Pix68zmjw>

This is how failed attempts are logged by vsftpd

fac_authpriv:Aug 9 16:24:42 web01 vsftpd[7255]: pam_ldap(vsftpd:auth): Authentication failure; user=xxxxx fac_authpriv:Aug 9 16:24:42 web01 vsftpd[7255]: pam_unix(vsftpd:auth): authentication failure; logname= uid=0 euid=0 tty=ftp ruser=xxxxx rhost=xxxxx user=xxxxx fac_ftp:Aug 9 16:24:44 web01 vsftpd[7255]: [xxxxx] FAIL LOGIN: Client "x.x.x.x" lev_notice:Aug 9 16:24:42 web01 vsftpd[7255]: pam_ldap(vsftpd:auth): Authentication failure; user=xxxxx lev_notice:Aug 9 16:24:42 web01 vsftpd[7255]: pam_unix(vsftpd:auth): authentication failure; logname= uid=0 euid=0 tty=ftp ruser=xxxxx rhost=xxxxx user=xxxxx lev_warn:Aug 9 16:24:44 web01 vsftpd[7255]: [xxxxx] FAIL LOGIN: Client "x.x.x.x"

Using dovecot-2.2.36-3.el7.x86_64 on CentOS7