First thing to consider are you signed up to the Auto Renewal as it will renew your subscription and payment ?
When it runs out use windows programs and features to uninstall avast Pro, you should be prompted if you want to drop back to avast free.
I haven't used Avast SecureLine VPN, as far as I'm aware it is possible to have it with avast free. I don't believe it is linked to any avast paid version, as it is a separate payment.