Searching “Vampire Dolphin movie” lead me to this wild plot synopsis for a movie called Reflecting Skin
"The film opens with Seth and his friends, Eben and Kim, playing with a frog Seth has found in the fields. The boys inflate the frog by inserting a reed up its anus and leave it by the side of the road. When a local English widow, Dolphin Blue, stops to inspect it, Seth shoots the inflated frog with a slingshot, causing it to explode over Dolphin. "
According to the plot, they suspect Dolphin of being a vampire. So close…