I tried mounting an NFS share over OpenVPN on a Linode VPS. At first I was getting this error:

# mount /var/www/virtual
mount.nfs: an incorrect mount option was specified

Digging into dmesg led to this error:

NFS: bad mount option value specified: vers=4

Apparently the linode kernel only has support for version 3, and thanks to this nfs mount error blog post at My Life as a Sysadmin, I was able to find a workaround:  /var/www/virtual nfs    _netdev,noatime,rw,vers=3 0 0