It could be diet related; maybe you're lacking in vital nutrients that you're not getting as a vegetarian. I think Atkins said something about a connection between good diet and teeth.
I don't know if there's any benefit to drinking baking soda with lemon juice, but if you're going to drink it, you should do it with a straw to prevent the abrasive quality from the baking soda and the acid from the lemon juice from eating away at your teeth and making the situation worse.