applying VPN to single program/application only

[dajhorn]

For an IRC client, the easiest solution is a dynamic tunnel over an SSH session.

First, at the command line on Linux:
# ssh -D 1080 vpn.example.com

Alternatively, in PuTTY on Windows:
Options -> Connection -> SSH -> Tunnels ->
Source Port = 1080, Dynamic

Second, configure the IRC client to use the SOCKS5 proxy at localhost:1080.

This configuration much easier to do correctly for one application than any kind of custom routing over a VPN.