I have attached a code sample for monitoring network traffic on a particular interface. Compiling and executing is very simple:
Near the top is a line that you can change the name of the interface being monitored:
You can change it to tap1 or tun0 or en0 or en1 or whatever is necessary.
This program displays ten samples taken one second apart, each sample displaying total packets in/out, total bytes in/out, and average bandwidth over the last second.
The source code should be very useful (to either of you) for modifying Tunnelblick to monitor network traffic.
The only unfortunate thing is that even after the VPN is disconnected and reconnected, the total packets and bytes are not reset until the computer is restarted. So this number will not represent "traffic since connected" but rather "traffic since boot." It's unfortunate, but I can't find a way around it. Unlike linux, Mac OS X does not appear to have a way to reset network statistics on an interface.
Don't know how much it will help, but hopefully it will some.
$ ./bandwidth
Packets: in = 2234504, out = 1435886
Bytes: in = 1.16 GB, out = 129 MB
Bandwidth: in = 0 B/s, out = 0 B/s
Packets: in = 2238104, out = 1436666
Bytes: in = 1.17 GB, out = 129 MB
Bandwidth: in = 2.59 MB/s, out = 21.4 KB/s
Packets: in = 2241670, out = 1437488
Bytes: in = 1.17 GB, out = 129 MB
Bandwidth: in = 2.57 MB/s, out = 21.7 KB/s
Packets: in = 2245206, out = 1438356
Bytes: in = 1.17 GB, out = 129 MB
Bandwidth: in = 2.55 MB/s, out = 24.3 KB/s
Packets: in = 2248688, out = 1439202
Bytes: in = 1.17 GB, out = 129 MB
Bandwidth: in = 2.51 MB/s, out = 22.3 KB/s
Packets: in = 2252218, out = 1439830
Bytes: in = 1.18 GB, out = 129 MB
Bandwidth: in = 2.55 MB/s, out = 16.6 KB/s
Packets: in = 2253080, out = 1439944
Bytes: in = 1.18 GB, out = 129 MB
Bandwidth: in = 629 KB/s, out = 3.01 KB/s
Packets: in = 2256370, out = 1440336
Bytes: in = 1.18 GB, out = 129 MB
Bandwidth: in = 2.37 MB/s, out = 11.2 KB/s
Packets: in = 2258644, out = 1440566
Bytes: in = 1.18 GB, out = 129 MB
Bandwidth: in = 1.64 MB/s, out = 6.36 KB/s
Packets: in = 2258788, out = 1440590
Bytes: in = 1.18 GB, out = 129 MB
Bandwidth: in = 100 KB/s, out = 2.57 KB/s
$ ./bandwidth
Packets: in = 2265334, out = 1443784
Bytes: in = 1.18 GB, out = 131 MB
Bandwidth: in = 0 B/s, out = 0 B/s
Packets: in = 2265598, out = 1444382
Bytes: in = 1.18 GB, out = 131 MB
Bandwidth: in = 7.2 KB/s, out = 419 KB/s
Packets: in = 2265984, out = 1445096
Bytes: in = 1.18 GB, out = 131 MB
Bandwidth: in = 10.9 KB/s, out = 517 KB/s
Packets: in = 2266332, out = 1445940
Bytes: in = 1.18 GB, out = 132 MB
Bandwidth: in = 9.41 KB/s, out = 600 KB/s
Packets: in = 2266680, out = 1446520
Bytes: in = 1.19 GB, out = 132 MB
Bandwidth: in = 10.5 KB/s, out = 409 KB/s
Packets: in = 2267136, out = 1447542
Bytes: in = 1.19 GB, out = 133 MB
Bandwidth: in = 12.3 KB/s, out = 723 KB/s
Packets: in = 2267366, out = 1447892
Bytes: in = 1.19 GB, out = 133 MB
Bandwidth: in = 6.1 KB/s, out = 248 KB/s
Packets: in = 2267686, out = 1448802
Bytes: in = 1.19 GB, out = 134 MB
Bandwidth: in = 9.35 KB/s, out = 634 KB/s
Packets: in = 2268218, out = 1449602
Bytes: in = 1.19 GB, out = 135 MB
Bandwidth: in = 16.5 KB/s, out = 571 KB/s
Packets: in = 2268566, out = 1450446
Bytes: in = 1.19 GB, out = 135 MB
Bandwidth: in = 9.46 KB/s, out = 596 KB/s
$ ./bandwidth
Packets: in = 3782, out = 3308
Bytes: in = 825 KB, out = 475 KB
Bandwidth: in = 0 B/s, out = 0 B/s
Packets: in = 3787, out = 3310
Bytes: in = 829 KB, out = 476 KB
Bandwidth: in = 3.92 KB/s, out = 132 B/s
Packets: in = 3790, out = 3312
Bytes: in = 832 KB, out = 476 KB
Bandwidth: in = 3.46 KB/s, out = 132 B/s
Packets: in = 3793, out = 3314
Bytes: in = 836 KB, out = 476 KB
Bandwidth: in = 3.41 KB/s, out = 132 B/s
Packets: in = 3796, out = 3316
Bytes: in = 839 KB, out = 476 KB
Bandwidth: in = 3.41 KB/s, out = 132 B/s
Packets: in = 3799, out = 3319
Bytes: in = 843 KB, out = 476 KB
Bandwidth: in = 3.54 KB/s, out = 480 B/s
Packets: in = 3802, out = 3321
Bytes: in = 846 KB, out = 476 KB
Bandwidth: in = 3.55 KB/s, out = 132 B/s
Packets: in = 3805, out = 3323
Bytes: in = 850 KB, out = 477 KB
Bandwidth: in = 3.55 KB/s, out = 132 B/s
Packets: in = 3808, out = 3325
Bytes: in = 853 KB, out = 477 KB
Bandwidth: in = 3.55 KB/s, out = 132 B/s
Packets: in = 3809, out = 3326
Bytes: in = 854 KB, out = 477 KB
Bandwidth: in = 866 B/s, out = 66 B/s