Thank you!

This works great! Thank you so much for this plugin. Made this account just to say so and to also assist other users in the future that are new to Nagios such as myself.

the 255 out of bounds messages is because you are not creating an _SNMP_COMMUNITY variable in the host definition. e.g.:

define host{
use generic-switch ; Inherit default values from a template
host_name FG80CXXXXXXX ; The name we’re giving to this host
alias Fortigate Firewall ; A longer name associated with the host
_SNMP_COMMUNITY public
address 192.168.1.1 ; IP address of the host
}

And then to check IPSEC VPN:
define service{
use generic-service
host_name FG80CXXXXXXXXX
service_description IPSec VPN Status
check_command check_fortigate_vpn!ipsec
contact_groups twilio_sms_alert
}