First you must generate a ssl certificate:
openssl req -new -x509 -nodes -out ftps.pem -keyout ftps.pem
Copy the ssl certificate to your vsftpd directory:
cp ftps.pem /etc/vsftpd
Add the following to vsftpd.conf:
ssl_enable=YES
rsa_cert_file=/etc/vsftpd/ftps.pem
force_local_data_ssl=NO
*Note ftps will only work with authenticated users, eg users with a valid account, not anonymous