Technology

The SSL/TLS encryption used on the web is designed to prevent that type of attack. It's called "asymmetric encryption", because a different key is used to encrypt and decrypt data. So each side has both a private and public key. You can broadcast your public key, and then anyone can use it to encrypt a message that only YOU can decrypt, with your private key. The private key never goes over the network, so the VPN provider can't intercept messages meant for you.

It's possible to spoof the server's certificate and make your browser think it's talking to a real server when it's not. However, this is difficult, because certificates follow a chain of trust, so they'd either need to compromise one of the certificate authorities (plausible for state-level actors), or compromise your web browser/OS so that it trusts a fake authority (definitely possible for a browser/OS vendor, but again, it would be noticed, especially in an open-source product like Firefox).

There are lot of enterprise products out there that perform this kind of "attack" so that businesses can monitor their network traffic. They do this by requiring adding their certificate authority to the list of trusted certs in the OS. This is often called "deep packet inspection". If you try to use these products without adding their certs, your browser will throw a fit every time you load a site because it will see that the certificate doesn't match.