Hi .. if you are using the ASA as the hub then I suggest you configuring the spokes routers with netflow. You can collect the netwflow information on a centralised server running netflow analyzer 5.0.
This tool can generate nice reports for bandwidth utilization ..etc. Now because you have VPNs and the traffic is encrypted you will not be able to see the applications been used but for tunnel overall utilization shoudl be OK.
http://manageengine.adventnet.com/products/netflow/
http://manageengine.adventnet.com/products/netflow/help.html
I hope it helps .. please rate it if it does !!!