When you say Juniper VPN connection, which Juniper product are you referring to? Juniper SSL VPN or one of the Juniper firewalls?
There is native support in the BB OS for Juniper SSG (ScreenOS-based) firewalls. There is no support for Juniper SRX (Junos-based) firewalls and it is impossible to configure.
The same applies to Juniper SSL VPN. If your BlackBerry is connected to the corporate network via a BES server, there is absolutely no need for connecting to a Juniper VPN (or any vendor's VPN for that matter) as there is already a secure tunnel from the device into the corporate network. You should be able to open a browser or other BB application and connect directly to internal resources. If you are unable to do so, please check with your BES admin to see why this isn't working as designed.
If you are a BIS subscriber, then unless you are running a ScreenOS based solution, I am sorry but there are no other options. There is a Junos Pulse client for the BlackBerry available via the App Market, but it is strictly for use for antivirus and other device management capabilities via the Juniper Pulse Moble Security Gateway product (separate from Juniper SSL VPN).