My mother has had this plant for at least 10 years now and it has bloomed only twice. The first time was about 2-3 years ago. It only has one flower, and it stems off of a leaf. It's quite large and the petals are soft and feathery. And it smells beautiful.
Or other cactus with nocturnal flowers. There are many. However, a common one asked about frequently is Epiphyllum oxypetalum. One of its common names is gooseneck cactus, referring to the bent floral tube which second photo appears to show.
If it's a "vine" of over 15 ft as you say, there are 2 likely candidates among commonly grown cacti. First is Selenicereus grandiflorus with rope-like stems; but that has flowers with hairy floral tube, so I think it can be ruled out. The other is Hylocereus undatus, which has jointed 3-angled stems, climbing by aerial roots. The one bit of cactus stem I can see in your pic is reasonably consistent with that.
