You could play with making the storage for this client “BIG_BACKUPS” a symlink to some other storage, no idea how well that would work though…
You might be able to do something with Storage Spaces, or creative choice of where you mount another hard drive in the file-system (in a folder rather than assigning a letter), maybe you can arrange that client’s backups to land there… I had concerns about Ur’s deduplication in that scenario, which may have been unfounded.
I personally solved having a client that creates massive backups by using a huge expandable DAS box (Drobo) my backup storage, NAS should work too… enough expandable capacity for whatever Ur wants to use even with one of the clients using lots.
I’m not convinced VMWare has anything to do with it per-se, my server and a number of the clients are under a VMWare variant too (ESXi), though with Workstation, you obviously introduce an extra bit of network config for the VM. How is it’s adapter set? NAT, Bridged,? If NAT (might be called shared or something, I’ve used so many virtualization solutions I mix up the terminology between vendors) you’ll need to do forwarding there too, I did that once myself for something or other years back. Bridged so the VM gets it’s own IP and looks like a whole separate machine to the network is generally simplest even if it’s not the default, unless you have a need to specify NAT or Host Only.