Unless you signed an agreement to that end, you don't have to do anything. Vested equity is yours. *Forcing* a buy-back is essentially repossessing your property. However, there are often reasons you'll want to negotiate a buy-back rather than hanging onto the equity:
First, you want to avoid bad blood in the relationship if possible - even if the company fails, the startup world is small, and your partner for this venture might return into your life later on in some other role.
Second, investors won't like absentee stakeholders. This will hurt the company's chances of securing the funding it needs to grow, and consequently increase the risk of your equity.
Third, the company can do things to dilute your stake after you leave, more than they would be able to do while you're there (even in ways they could unilaterally dilute you while there, they wouldn't want to do it because it would probably cause you to become disgruntled at the unfair treatment).
It's your choice (if you didn't sign an agreement with a buy-out clause), but if you're being offered a fair buy-out, it's usually a good idea to accept. It recoups the value of your time spent there, removes the risk, and allows the company to allocate an adequate amount of equity to your successor. If the company succeeds, you'll still gain prestige from being associated with it, which will help you in your next venture even if it doesn't result in a big cash-out on this one.
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