Ok thanks for your reply.
I've solved the problem by adding my local DNS to the VPN.
With some modification to named.conf (setting the local DNS as a slave for our vpn domain so that it forwards all requests concerning the vpn to the right DNS) I was able to resolve names on the VPN
Now the problem was to make samba browsing work which seems to be difficult (and maybe not possible) because we can't broadcast over the vpn (routed openvpn)).
Anyway I think we'll go for NFS sharing (I've noticed some speed differences and NFS seems to be faster )
Ho and then any links to some open source or freeware windows NFS tools? (I've tried omninfs from M$ but this is not only for NFS and I'd prefer use a lightweight tool since some of the windows computers are olds and slow