To fully explain this we need to look at how tor wooks for the tor + vpn option.

If you're using tor on its own your traffic will generally take 3 hops to the light web at least.

You know the ip of all the hops and where you want to go.
The first node knows your ip and the ip of the second node
The second node knows the first nodes ip and the ip of the third node
The third node knows the ip of the second node and the ip of the website you want to connect to.

Now, if you don't use a vpn, people can trace where you're trying to access if they can see the logs of the first and third node.
If you're vpn don't keep logs and have no backdoor for the government then your anonymity is enforced by no one being able to track you if you route your traffic this way.

It is also possible to chain vpns or have it so that you use a vpn to connect to a vps to process your requests through tor..

Using tor itself means you may face connectivity problems. I did a speedtest of tor a while ago and got 12mpbs which is a bit like a speed between 3 and 4G mobile networks so if you don't have to be completely anonymous for some tasks you may get away with just a vpn but note that law enforcement are able to track patterns in people and it isn't hard to do so be careful what you think you're safe doing without tor if you want to be completely anonymous.