https://hedberg.io/edgerouter-ipsec-vti-dynamic-address-script/

dynamic-vti.sh

#!/bin/vbash
# /config/scripts/dynamic-vti.sh

# configure vti address based on dynamic dns address

NEW_IP=$(getent hosts yourdomain.com | awk '{ print $1 }')
CUR_IP=$(cat /config/config.boot | grep site-to-site -A1 | grep peer | awk '{ print $2}')

echo "CURRENT: $CUR_IP"
echo "NEW:     $NEW_IP"

if [ "$CUR_IP" = "$NEW_IP" ]; then
   echo Addresses the same
   exit 1;
fi

echo "CHANGE NEEDED"
      
source /opt/vyatta/etc/functions/script-template
                                                
configure
delete vpn ipsec site-to-site
set vpn ipsec site-to-site peer $NEW_IP                       
# Paste the rest of your config here. 
# For example:
#   set vpn ipsec site-to-site peer $NEW_IP authentication pre-shared-secret xxxxx"
# Make sure you include all config items needed to set up your connection
commit                                                    
save                                   
                                       
exit              
                  
restart vpn 
ping 192.168.x.x # To initiate the link (create traffic)