What is it you mean exactly when you say you can't 'write'?
If I assume that you are able to read files, via the VPN, and from a shared folder on your work PC, but not write to them, I would say that the VPN tunnel is working 100% and you can pretty much rule that out as the cause of the problem.
this is because even reading files from a windows/unix share requires SOME 2-way traffic - if you are able to read files then data is passing both ways happily.
It may be purely a case of (most likely) NTFS permissions which are blocking your user/logon from writing files.
Please be sure to provide more info, i.e. OS, VPN etc and we'll see if we can be of more help.
Best Regards & Good Luck
Terry-Lee Blay
MCP, A+, Net+, Server+, APS, IBM Thinkpads/Portables
webwarrior@angelfire.com