I would imagine this varies massively between customers.
This would very much depend on:
- what your clients were doing over the VPN
- if the VIA client was configured to split tunnel traffic or not.
The VIA client sends the controller a keep-alive message every 15 seconds. So, with that in mind, the bandwidth the client itself uses is minimal.
Does that answer your question?
Or are you wanting to know about the overhead that the VIA client adds to the traffic that flows over the VPN? In which case, it's just IPSEC so (quickly googles) it adds about 10% give or take a few percent.