The most common causes for papule or vesicles on the inner side of the lips are:
From the information that you have given it seems like you are having mucocele.It is a benign condition usually caused due to trauma or lip biting.Most of the time it resolves on its own.If not,surgical removal with Co2 laser
is the better option.
Kindly consult dermatologist
to rule out any malignant conditions and further course of treatment.
Hope it helps.