Finally had another windows 7 64 bit pc to play with and this didn't stop transferring data.
So it could be the network drivers for the intel pro 1000pt or something else.
Time for more diagnosis.
This was the main reason why I didn't report it.

Now testing with a wireless card too see what happens.

Well with the wireless I did get the odd disconnection message otherwise it hung the same way.
Nat Gre session time-outs happened during this period as did fraggle attacks.
Both stopping as soon as I stop using the vpn.

Oops forget the fraggle attacks of course I forgot to change the ip address for the logging software when I changed adaptors

Noticed that Vigor have updated there vpn client. Same problem.
Now disabled udp flood defence as someone else has suggested.

No difference data stops flowing after 15 minutes.

Used wireshark and data is still being sent via vpn but I can't browse or ping anywhere. Very strange indeed.