Once the VPN connection is established, two-way communication is possible. But as for backup - how much data are we talking? You need to size that aspect first. Your total speed over the VPN will be throttled by the broadband (?) connections on the upstream side. Maybe 700Kbps if you're lucky? If you achieved 700Kbps, the maximum theoretical data transfer upstream would be about 7GB, and that's hammering the connection 24 hours.
To get back to your question - how to do this - what backup software are you intending to use?