I assume we're talking about configuring the OpenVPN client here. I don't understand why it has to be TAP. Are you saying the OpenVPN server you're using is itself constrained to TAP?
A TAP VPN is bridged between the local and remote networks, making *everything* accessible, by design. It's why you don't use a TAP VPN when you need to filter access. And since Merlin doesn't support user-defined VLANs natively, you'd have to consider using a third-party scripting solution. And the OpenVPN TAP client of the GUI is going to bridge the private network (br0) to the VPN, which would require additional manipulation to rebind it to the new VLAN.
So like a lot of things, it can probably be done, but NOT easily via the GUI.
Of course, use of a separate router for this VPN would solve the problem too.