Gather everything you signed, they signed and go talk to an attorney (google employment contract attorney and call a few) for an hour or two. Don't sign anything or agree to anything via email. The attorney can help you draft a response. It's worth the money (300-500 bucks maybe) just for the education you will get.

Finally, never have a conversation like the one you did two weeks before vesting. Hard lessons are learned in startups and I'm sure this is one you won't easily forget. My email is in a profile if you want to chat about this more. I've been through similar stuff.

why would you tell them that? Did you have any personal guarantees to pay back or something?