Moderate vascular plethora of the lungs means that the blood vessels, namely the arteries and the veins, are prominent. Depending on the clinical context, it might suggest the presence of some diseases like heart failure or pulmonary hypertension or occasionally asthma. It could also be a normal variant and might not suggest any disease process. It would be inappropriate to interpret the chest X-ray findings in the absence of a clinical history and I would infer one thing or the other based on the description you have provided. I advise that you talk to your physician more about this and get your concerns addressed.