VPN Security Vulnerabilities

Hi

I am currently nearing the end of my dissertation for my degree. I am investigating into VPN security and the different protocols used to create a tunnel.

I'm wondering if anyone can shed some light onto the possible security vulnerabilities posed by each solution (i.e. SSL, IPSec, PPTP etc). For example, SSL could be prone to a 'man in the middle attack' so how could this be prevented?

I hope this makes sense
Cheers
Jamie