Can vitamin d insufficiency cause sore, bleeding gums?

No: vit-c deficiency can cause bleeding gums. vit-d deficiency causes rickets in children and osteomalacia in adults which is a failure of calcification of cartilage and bone.

Vit D Deficiency: No. You're thinking of Vitamin C deficiency. Bleeding gums usually mean gum disease (bacterial), although in rare circumstance can be caused by blood dyscrasia or systemic disease. Please see your General Dentist or preferably a gum/bone specialist, a Periodontist, to begin the differential diagnostic process.