I have looked closer at my vsftpd setting:

There is no specific mention of this in the setting file, and the default is 0 (unlimited) for both per ip and general max connections.

you also asked for Iptables output, here it is:

root@XXXXXXXX:/etc# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            ctstate RELATED,ESTABLISHED 
WHITELIST  all  --  anywhere             anywhere            
BLACKLIST  all  --  anywhere             anywhere            
CHECK_SSH  tcp  --  anywhere             anywhere            tcp dpt:ssh ctstate NEW 
CHECK_FTP  tcp  --  anywhere             anywhere            tcp dpt:ftp ctstate NEW 
CHECK      all  --  anywhere             anywhere            

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Chain BLACKLIST (1 references)
target     prot opt source               destination         

Chain CHECK (1 references)
target     prot opt source               destination         
fail2ban-ssh  tcp  --  anywhere             anywhere            multiport dports ssh 
fail2ban-vsftpd  tcp  --  anywhere             anywhere            multiport dports ftp 

Chain CHECK_FTP (1 references)
target     prot opt source               destination         
           all  --  anywhere             anywhere            recent: SET name: FTP side: source 
DROP       all  --  anywhere             anywhere            recent: UPDATE seconds: 60 hit_count: 4 name: FTP side: source 

Chain CHECK_SSH (1 references)
target     prot opt source               destination         
           all  --  anywhere             anywhere            recent: SET name: SSH side: source 
DROP       all  --  anywhere             anywhere            recent: UPDATE seconds: 60 hit_count: 10 name: SSH side: source 

Chain WHITELIST (1 references)
target     prot opt source               destination         
ACCEPT     all  --  xxxxxxx       anywhere            
ACCEPT     all  --  xxxxxxx/24      anywhere            
ACCEPT     all  --  xxxxxxx/24      anywhere            
ACCEPT     all  --  xxxxxxx        anywhere            
ACCEPT     all  --  xxxxxxx        anywhere            
ACCEPT     all  --  y.y.y.y       anywhere            

Chain fail2ban-ssh (1 references)
target     prot opt source               destination         
RETURN     all  --  anywhere             anywhere            

Chain fail2ban-vsftpd (1 references)
target     prot opt source               destination         
RETURN     all  --  anywhere             anywhere  
________

PS: y.y.y.y is the ip in question, which is ok, as long as it is white-listed.