Crypto Protocol Question - Can your ISP reverse engineer crypto your VPN uses?

I use a VPN because i don't like my ISP spying on me like they have in the past. I used the program mIRC once, and the ISP sent me an email saying i was infected with a botnet. They said I had to get the computer checked by professionals and had to email them back saying that we had fixed the problem. After that little incident I decided to use a VPN for privacy.

My question is this: Can the ISP break the VPN's cryptography if I use the VPN to fetch my email, which is provided by my ISP?

I would think that they in theory could crack the encryption since they would have the plaintext emails as well as the cyphertext that my VPN converts it to.

Also, if this is possible, what would be a good way to fix this? I wonder if disconnecting from the VPN then reconnecting would change the encryption key between me and the VPN?

Thanks in advance!

Secure algorithms are not vulnerable to the type of attack you're describing. Knowing the plaintext and the ciphertext doesn't allow an attacker to determine the key.

Even if the algorithm you're using is not secure, it's not likely that your ISP would invest the effort required to determine your key.

thanks for clearing that up for me. i am glad that this is the case, since i hate the isp's spying policies...