Yes, 100% control of 100% clients (If the client approves of it, and most do) ... We do this for remote support for most of our clients, where we have setup their IT infra... The VPN technology used is same for most, but yes, some are different .. But, no host ed VPN, for sure ..
We are comparatively a smaller company, with a comparatively smaller client-base .... For, most parts, one person from our end, is connected to just one client.. However, I think multiple simultaneous VPN connections have been made... I recollect, a colleague doing something with routing table to accomplish this ...