This assumes your main gnome Login keyring is automatically unlocked during login from your DM, or your keyring's password has been set to empty (the latter being common with autologin).
To store secrets:
$ secret-tool store --label="VPN Username" vpn-store username
<enter username here>
$ secret-tool store --label="VPN Password" vpn-store password
<enter password here>
$ secret-tool store --label="VPN URL" vpn-store url
<enter url here>