Review FortiNet Cookbooks and documentation
diag debug disable
diag debug reset
diag vpn ike gateway flush name VPN-NAME (reset the SA)
diag vpn ike log filter name VPN-NAME
diag debug application ike -1
diag debug enable
di de di (turn off debug)
You may need to restart the process IKED
daig sys performance top
diag sys kill 11 PID