Answer by atrus for OpenConnect: Setting default routes

Here's a script which does split-tunnelling on both IPv4 and IPv6 networks (based on the script Aditya K provided, which still allowed caused all IPv6 traffic to be routed to the VPN). Note also that the setting of CISCO_SPLIT_INC_${N}_MASK variables no longer appears to be needed):

#!/bin/bash
#

# Routes that we want to be used by the VPN link
ROUTES="162.73.0.0/16"
ROUTES6="1ef2:23e:5a4::0/48"

export CISCO_SPLIT_INC=0

# Create environment variables that vpnc-script uses to configure network
function addroute()
{
    local ROUTE="$1"
    export CISCO_SPLIT_INC_${CISCO_SPLIT_INC}_ADDR=${ROUTE%%/*}
    export CISCO_SPLIT_INC_${CISCO_SPLIT_INC}_MASKLEN=${ROUTE##*/}
    export CISCO_SPLIT_INC=$((${CISCO_SPLIT_INC}+1))
}

export CISCO_IPV6_SPLIT_INC=0
# Do the same for IPv6
function addroute6()
{
    local ROUTE="$1"
    local NET="${ROUTE%%/*}"
    local MASKLEN="${ROUTE##*/}"
    export CISCO_IPV6_SPLIT_INC_${CISCO_IPV6_SPLIT_INC}_ADDR=$NET
    export CISCO_IPV6_SPLIT_INC_${CISCO_IPV6_SPLIT_INC}_MASKLEN=$MASKLEN
    export CISCO_IPV6_SPLIT_INC=$((${CISCO_IPV6_SPLIT_INC}+1))
}

# Old function for generating NetworkManager 0.8 GConf keys 
function translateroute ()
{
    local IPADDR="${1%%/*}"
    local MASKLEN="${1##*/}"
    local OCTET1="$(echo $IPADDR | cut -f1 -d.)"
    local OCTET2="$(echo $IPADDR | cut -f2 -d.)"
    local OCTET3="$(echo $IPADDR | cut -f3 -d.)"
    local OCTET4="$(echo $IPADDR | cut -f4 -d.)"

    local NUMADDR=$(($OCTET1*16581375 + $OCTET2*65536 + $OCTET3*256 + $OCTET4))
    local NUMADDR=$(($OCTET4*16581375 + $OCTET3*65536 + $OCTET2*256 + $OCTET1))
    if [ "$ROUTESKEY" = "" ]; then
        ROUTESKEY="$NUMADDR,$MASKLEN,0,0"
    else
        ROUTESKEY="$ROUTESKEY,$NUMADDR,$MASKLEN,0,0"
    fi
}

if [ "$reason" = "make-nm-config" ]; then
    echo "Put the following into the [ipv4] section in your NetworkManager config:"
    echo "method=auto"
    COUNT=1
    for r in $ROUTES; do
        echo "routes${COUNT}=${r%%/*};${r##*/};0.0.0.0;0;"
        COUNT=$(($COUNT+1))
    done
    exit 0
fi

for r in $ROUTES; do
    addroute $r
done

for r in $ROUTES6; do
    addroute6 $r
done

exec /etc/openconnect/vpnc-script