A VPN is not simply an "ip changer", I do believe that a VPN also encrypts all of your traffic so that your ISP and people on your network can't see what you're doing. PID was explained very well in the post above, no need to re-explain it.
I've been botting for years and never once been banned for it. Maybe it's because I only run 1 account on the same IP address or I look at the bot every 30 minutes or so and chat to some of the people around where I'm botting.
There (hopefully) is no such thing as a one time fee VPN.
I recommend purchasing a low powered VPS (for about $5 a month) and installing OpenVPN on it.