Logging into a VPN server requires a username and password. If you are going to enter VPN setup into anything it is going to ask you for a username and password.
There doesn't seem to be any way to say "I want to give you the VPN Server address, but I want you to ask the user for their details when they try to log into it.". Sorry.
I haven't tried this for a VPN profile but have done variables with an Exchange email profile. The user logs into the My Devices web portal with credentials that are then used for the installation of the profile substituting their real information for the variable. See these articles: