As of this writing, there are 54 episodes of the show, and 11 books some of those are based on. I mention this because The Darkest Evening (TDE) just so happens to be one of the very few stories that has a book of the same name. I phrased it that way because while I haven't read TDE I assume the mystery is a bit more... mysterious.
This case should've been solved in ten minutes, within the show that is; realistically in about about a day. All they had to do was get the victim's phone record to see who last texted her and that would be it. That's usually the first thing the team does and unless I missed it that conveniently wasn't done this time, they only mentioned that her phone wasn't on her.
Also, what was Vera doing out there? That wasn't her regular way home, and she wasn't going to visit those relatives we only just learned about; otherwise we would've heard about them sooner. I haven't read many of the books, but I have watched every episode to this point (most more than once) and I don't recall any other surviving family member mentioned before, so throwing them in the 9th book/50th episode feels a bit tacked on.
Other than that it was the usual Vera storyline: great scenery, everyone lying as if they were all practiced sociopaths, and the interviews stopping several questions sooner than they should have. TDE however seemed more Agatha Christie and less Ann Cleeves. Cheers.