This question already has an answer here:
Yesterday my ISP dropped my connection for several hours. This idea came to me during that time.
Well, I don't know much about these things, so I better re-phrase to be crystal clear and mention some things that might be important:
System: Linux Mint 18.2, kernel 4.10
VPN: Private Internet Access
OpenSSL: Version 1.0.2
I only imagine that the ISP could interscept the initial handshake and by that possibly decrypt the communication? Is something like that possible?