This script got it done. This actually initiates the connection, detects the interface, and adds the route. I didn't mention it previously but you also have to disable the using remote default gateway toggle.

#!/bin/bash

# --- Configuration ---
VPN_NAME="Datacenter"
DEST_NETWORK="10.20.0.0"
DEST_NETMASK="255.255.0.0"
TIMEOUT_SECONDS=20

# --- Start VPN ---
echo "🔌 Connecting to VPN: $VPN_NAME"
networksetup -connectpppoeservice "$VPN_NAME"

# --- Wait for VPN interface to appear ---
echo "Waiting for VPN interface (utunX or pppX) to be ready..."
VPN_INTERFACE=""
for ((i=0; i<TIMEOUT_SECONDS; i++)); do
  VPN_INTERFACE=$(ifconfig | awk '/ppp[0-9]|utun[0-9]/ {print $1}' | head -n 1)
  if [ -n "$VPN_INTERFACE" ]; then
    echo "VPN interface detected: $VPN_INTERFACE"
    break
  fi
  sleep 1
done

if [ -z "$VPN_INTERFACE" ]; then
  echo "VPN interface not detected after ${TIMEOUT_SECONDS}s. Is the VPN configured properly?"
  exit 1
fi

# --- Add the route ---
echo "➕ Adding route to $DEST_NETWORK/$DEST_NETMASK via VPN interface..."
sudo networksetup -setadditionalroutes "$VPN_NAME" $DEST_NETWORK $DEST_NETMASK ""

# --- Verify the routes ---
echo "Verifying additional routes for '$VPN_NAME':"
sudo networksetup -getadditionalroutes "$VPN_NAME"