As the child is under the care of a qualified doctor, you need not worry if both the items are prescribed by the same doctor. However, if the second one is prescribed by a new doctor, you should apprise him of the fact of taking Vitamin drops as per the previous doctor's suggestion. There are certain cases when generally harmless and non-interfering vitamins also interfere with some medicines. So it is always a must that whenever you meet a doctor, you should apprise him of all the medicines being taken by the patient general and also for that specific illness if any.
It is also better to ask for any regulation o avoidance of any other medicine or food in case you find a not-so-common medicine prescribed to you. Doctors in their crowded busy schedule and routine prescription can forget to tell you same. It happened to us when our son was small. It was only when I specifically asked, the doctor got alerted and told me such and such should be avoided. There was one medicine the child was taking and it was not to be taken with the new medicine prescribed. The doctor said sorry and appreciated me for asking a very relevant question. From then on I made it a habit to ask doctors detail, even when it may some elementary to them sometimes.
So your question is not wrong, but in future get it cleared then and there with the doctor himself.
