Negotiating at the endgame is way, way too late. You need to negotiate this at the beginning. Specifically, make sure there are no liquidation preferences. Those preferences might say that, if the company is sold for less than $X, the investors get their investment out in whole, and then whatever is left is split proportionally among the holders of vested options.

I was screwed by this once, and have since pressed to have liquidation preferences removed. Once was in a B round, and the VCs dropped them completely. Another time I was part of the founding team and successfully insisted they be removed in the A round.

I've seen VCs argue in favor of liquidation preferences (there was an HN discussion pointing to a discussion on avc.com). Understandable from their point of view. You can fight back, but if the investors won't give up on them, you obviously have a hard choice.