Your basis is zero, so you have nothing to lose except the risk.
Actually with Restricted Stock Rights your basis is the market value on the date the shares vest, that value is considered ordinary income to you on that date, and you owe tax on that amount when you file your income tax return the following April.
Fun with taxes! BPGUI is right. What do we always say, 'check with a tax professional'? Sloppy language on my part. In any case, your out-of-pocket expense to acquire these shares is zero
Also, companies have a habit of doing a standard withholding (not using your stated number of exemptions) for these transactions, so it may affect how you want to fill out your tax withholding forms for your regular income in the year of vesting.