All of your client TVH interaction on the clients will be with the server, so the only interactions with the outside world [e.g. streaming IPTV] will be via the server & thus via the VPN.
Any other stuff you do on the clients that is not using the TVH client will not use the VPN[1], for example if you installed an IPTV streaming add-on on a client, it would not use the VPN.
fn1. Unless, of course you installed routes on the clients to go via the server [and thus its VPN].