Take a look at the sentence again:
Jean always nibbles around the pit of a cherry, just as if she were eating
a peach, because she is vain about her smile, she
doesn't want to chip a tooth on the hard center.
The comma between smile and she is completely correct. Because she is vain about her smile is a subordinate clause. A subordinate clause cannot stand alone as a complete sentence. You must attach it to a main clause to finish the thought. She doesn't want to chip a tooth is the main clause that completes the idea.
Go back to the sentence to try again.