Access internal Lync 2013 server from third party vendor site

Hi,

We have LYNC 2013 installed on-premises.  We have a third party vendor that provide some of the IT related support.  They need to be able to use Lync to provide support to internal users.  What would be the best option to achieve this?

We have thought about two options:

1) Have a virtual machine running at vendor site with site-to-site VPN configured.  Have Lync installed on the VM.  However this may not work as the VM is not part of the internal domain

2) Have a virtual machine running at our site , vendor RDP onto the machine and use Lync.  The concern is that Lync 2013 client won't work properly under RDP connection and would still face the same audio/video quality issue due to a VPN connection required for the RDP.

Any help would be much appreciated.

Thanks

Winnie