Regarding my issue, I've found the following post.
http://serverfault.com/questions/238695/how-can-i-set-the-metric-of-a-manually-added-route-on-windows
It means the problem is due to Windows.
Windows adds metric value instead change it. The workaround is to change metric manually on Windows TAP interface but it's a problem with a lot of clients without Group Policy etc.
The following command also helps but you have to know interface number.
Set Windows metric to 0, then OpenVPN metric + Windows metric = OpenVPN metric
netsh interface ipv4 set interface 25 metric=0
It only works via command line with administrative privilages.
From Windows control panel is not possible to set it to 0 because Windows says that possiible range is between 1 and 9999.
is it possible to add the above command as standard command like "route.exe ADD....." after established VPN connection?
