(You're alone, you didn't have a bag or something to throw up in or it just happened too quickly, and the driver didn't even notice that you threw up. Yeah, there are those barriers between you two and he just didn't notice.)
a. Do nothing. Pay and get out of there.
b. Tell him what happened and apologize profusely, pay and get out of there (letting him take care of the mess or whoever else when he gets back to his base).
c. Apologize etc and at least offer to clean it up. (Perhaps as a cabbie they expect these things to happen and keep towels somewhere)
d. Other - Maybe I'm not imaginative enough, can't think of other options now, but if you have one, by all means.. ;)