I have a mildly-enlarged spleen and have been vaping marijuana daily for about 3 months. just found out about the spleen. could that be the cause?

Doubt it: Your spleen is the largest lymph node in the body and as such enlargement usually reflects a response to an infectious process. A common one would be mono but there are many others. Inhaled THC is not expected to trigger such a response.

Not likely: Not likely to be related. Talk to your doc about the need for further tests. Sometimes surveillance is needed. Keep a log of your symptoms as well. Otherwise, always worth stopping something like vaping unless you are treating a different symptom or condition.