Given that the title of the show was "Buffy The VAMPIRE Slayer," I'd have to say that the vampires in the Buffyverse got the short end of the stick in the "badness" category, especially after Season 3. I don't think that that was any attempt to belittle vampires as much as it was to introduce a little variety onto the show.
Having said that, however, I do think that vampires became less and less menacing as the show progressed, to the point that, by the end, they were more apt to be some other demon's minion or hired muscle than to have the kind of status that Luke and Darla had in Season 1.
I agree. I don't think they were intentionally trying to trivialize vampires, but I do think they were ridiculously sidelined midway through the series, to the point that I actually thought the title of the show became sort of a misnomer (in the vein of
"New" Girl). And I also felt vampires didn't need to be pushed to the side as much as they were, especially considering that we have seen, though vampires like Angel and Spike, that some are indeed exception and exceptionally strong. I mean, I know Angel is meant to be unique, but the way he was portrayed, especially on ATS, it seemed like he was much stronger than most demons. Or at least, he seemed to be able to defeat many demons with relative ease. Basically, we know that not all vampires are created equal (I mean even plain old Xander was a slap-fight match for vamp Harmony). Thus, I think they very well could have thought of something new and creative to do with vampires (instead of just the brainless, beastly Turok-hans). I'm not saying
all vampires suddenly needed to be portrayed as being as strong as Angel, since only the Big Bad would have to be another unique vampire.
Most of the Big Bads of the latter seasons were problematic to me because they were overly ambitious, and poorly executed. Specifically, I have Adam, Glory, and The First in mind. Conceptually, these were all great ideas, but none seemed to work quite as well as they could have.