You never “disappear” on the Internet because, well… you’re on the Internet. What you hope to accomplish with a VPN, however, should dictate how you use it.
Using a VPN creates an encrypted tunnel between you and the VPN service provider. Your ISP can see that you are connected to the VPN provider’s IP address and the bandwidth you are using to and from this address (through the tunnel), but cannot see anything else. The VPN provider still sees all your traffic to the Internet.
Your “public” IP address to the rest of the Internet now becomes the public IP provided by the VPN provider. This address likely will be provided through Network Address Translation (NAT), so your “public” IP will be shared with other VPN users.
As others have stated though, there are other ways that sites can track you, i.e. login accounts, cookies, browser fingerprinting, etc. So, if you’re using services and accounts tied to you personally, then you’ve lost any shred of anonymity. Who knows how much of this information about you is shared between sites and services.
So, if your objective is simply to hide your traffic from your ISP, when torrenting movies for example, then use the VPN as loosely you please. Your ISP can send warning letters about copyright violations only if they can see what you are doing. The VPN provider certainly won’t care and, if not keeping logs, cannot give you up to anyone requesting your identify.
If your objective is to be anonymous with no activity being traced to you personally, then you should be more guarded. How?
- Pay for the VPN service anonymously (e.g. using a gift card) and never, ever, ever use the VPN service from your home. Always use it from a public place.
- Create a virtual machine (VM) on your personal computer using something like the free VirtualBox, or commercial VMware products, and install a non-intrusive OS like Linux.
- Configure the VM to acquire its own IP rather than share the hosts IP.
- Install a security-focused web browser and never login anywhere with an account tied to you.
- Install The Onion Router (TOR) and use it after connecting to your VPN. TOR uses a custom version of Firefox I believe, but you can have both.
- If you are really paranoid, then you can create a snapshot of your VM immediately after installation, and restore it after each use so that your VM is always in a freshly-installed state.
Your path to the Internet now is…
Anonymous VM (with freshly-installed OS) => Public Internet Access => Anonymous VPN => Anonymous browser and/or TOR browser => Internet
I am no security pro or hacker, but these steps should keep you relatively anonymous.
To be honest though, I doubt the above would protect you completely if some three-letter-agency is tracking you.