Well, yeah. Just like you leak your encrypted password to the internet every single time you log into a website.
>What if there is an undiscovered or undisclosed vulnerability in the encryption?
lmao, if aes-256-cbc is broken then LastPass is probably the least of anyone's concerns. This happens to also be one of the more difficult AES modes to screw up.
>What if last pass isn't using encryption as secure as they claimed?
Shit, if that was a real concern you would have to be a complete idiot to use LastPass in the first place.