Hi.. Thanks for the query... Ulcer at the edge of the lower set of the denture can be most probably due to any sharp elevation over the edge of the denture and as it rubs over the mucosa it can cause tear of the mucosa and ulcer formation.. Applying Vaseline alone is not sufficient although small amounts will not harm if ingested.. So first of all you should get your denture examined by your Dentist and if there are any sharp edges they will be smoothed as well as if the flanges of the denture are oversized then they will also be trimmed.. For now avoid wearing denture as well as start applying a mild steroid oral paste like Kenalog over the ulcer.. You should also do warm saline gargles and do cool compresses over the ulcer to reduce inflammation and pain. You can also suck ice pops.. Avoid hard and spicy foods .. Hope this helps.. Regards..