The traffic enters the VPN at your computer, got nothing to do with the ISP. That's the entire point of having a VPN, to prevent eavesdropping between you and the VPN provider.
No no no.
You need an active Internet connection to use a VPN. So you need an ISP. No ISP, no VPN.
The way it works is as follows (in a very basic summary),
Your PC connects to the Router and obtains an IP address. Your Router acts as your gateway, (with an IP like 10.28.47.1, as an example.)
Your router connects to your ISP's servers, which then grant access to the Internet.
You start your VPN, which creates a tunnel in the already existing connection from your PC to the relevant vpn servers.
The point is, anything in the tunnel is safe from inspection BUT the PC and server are not inherently secure.
Let's say GCHQ want to monitor you, all they need to do is get your gateway and/or Routers MAC address, which they can get via court order from your ISP, and they're in your home network. Which means they can directly monitor your PC, thus the VPN is useless because they're not looking at the data coming out, they're looking at the source.
This is the point I was making, if your home network and PC aren't secure, so that can include every single device in your house that connects to the Internet or Wi Fi, every single one of them is a means to ignore your VPN.
The vast majority of people aren't knowledgeable enough to secure a modern house network. For example, on mine I have my TV, Xbox, PlayStation, phone, alarm system, boiler, tablet, laptop, computer and a lamp. Luckily I work in IT and I'm fairly brushed up on Network security, but the vast majority aren't so telling them to use a VPN is giving false confidence when they could enter your network through your fucking fridge soon.