I am no expert but I suppose if they start new company (and they are successful) you could sue them. The prior work that was justification for the vesting was m

I am no expert but I suppose if they start new company (and they are successful) you could sue them. The prior work that was justification for the vesting was market-tested by the startup. The result was negative but I suppose your share was not conditioned like this. Also the pivot happened while you were still part of the company, right? In general I would say the best option for both sides is to reach a consensus somewhere in the middle.