Previously in Varney the Vampire: Charles resolves to do something stupid.
Chapter 25 (THE ADMIRAL'S OPINION. -- THE REQUEST OF CHARLES.) is the second digression in six chapters. Really, JMR? But to be fair, this one kind of has to do with the story, in that rather than someone reading an unrelated tale, the Admiral relates it to Charles. I'm not going to quote because I didn't really find much to nitpick, but the gist is that a giant stranger appears, seemingly out of nowhere, on a ship during a sea voyage. He summons up a storm, drinks all their coffee, and disappears again.
In his annotated edition of Varney the Vampire, Curt Herr notes that there are parallels between the Admiral's story and the episode on the Demeter in Dracula. I haven't studied the texts extensively, so maybe there are more intricate details I'm missing, but "they're both creepy stories about a supernatural figure on a boat in books about vampires" comes across as more coincidence than anything.
When I read the footnote, I expected the twist at the end to be that the stranger on the boat was Varney. The storm thing is hard to explain for a vampire, but as JMR hasn't fully established the rules of vampirism at this point in the narrative, I can't discount anything for sure. But the eating and drinking definitely proves that the stranger on the boat isn't a vampire.
So in the end, the story is really not related to Varney any more than the penny dreadful Flora read in Chapter 19 -- merely better integrated, and (I think) an overall more interesting story. Entertaining enough, but I'd like to get back to Varney.