To enable multiple VPN users with different credentials, find and replace these lines in the script:

cat > /etc/ppp/chap-secrets <<EOF
# Secrets for authentication using CHAP
# client server secret IP addresses
"$VPN_USER" l2tpd "$VPN_PASSWORD" *
EOF

with the following:

cat > /etc/ppp/chap-secrets <<EOF
"user1"  l2tpd  "password1"  *
"user2"  l2tpd  "password2"  *
EOF

-----------------
 IMPORTANT NOTES
-----------------
1. Make sure there is NO SPACE after both EOF above!
2. This example shows user1 with password1, and user2 with password2. You can add more users, use one line for each user.
3. The characters \ and " within your username/password MUST be escaped as \\ and \", respectively.