I have only two ideas, how to start error search:
1. Try to connect to the VPN -Samba-shares using a Desktop PC
2. Do your Android-Device offer a native VPN-support (So does mine!) Maybe the App you're using to connect to the samba-share does not use the VPN-Tunnel, because OpenVPN does not provide it to other apps? I once had a similar issue, when i tried to connect to my home VPN-Network, while i was in another WLAN, where the IP-Adresses where the same as in my home-Network (192.168.1.1 was the router, 192.168.1.99 was the NAS) and i was routed to the NAS in the local network although the VPN-connection was enabled! I changed the IP of my NAS at home and the problem was gone. Don't know if this helps...