I watched this under the name _Vampire Ecstasy_, which makes slightly more sense than The Devil's Plaything. The movie feels like it should be a period piece, but it's set in the time it was made: the '70s.
So, this is a lesbian-vampire movie - my favorite horror subgenre - but it's very light on the vampire aspect. There's no real plot to speak of: people are at a castle... for reasons, and there are strange women there who gather, at night, in a section of the castle, and they dance erotically to the sound of a drum. I hope you like that drum beat, because you're going to hear it a lot, and it's the only music in the entire movie - aside from white noise running through a flanger, and minimal synth at the very end.
For even more reasons, when the semi-vampires dance nude, it makes everyone at the castle horny. That's basically the movie.
The acting is terrible. There is almost no music. The dialog is weak. The lighting is good some of the time. The camera work might be the main highlight, but that's not saying much. Really, this movie is here for one purpose, and you're either okay with that or you're not.
I think the influence from Rollin, and Franco is very obvious. It's as if Joe Sarno watched their movies and said "I can do marginally better than that."
There is only one real vampire in the movie. The weird cult ladies behave like vampires - they hiss, they hate garlic crosses - but they aren't actually.
Two of the most unintentionally-funny moments are when our story's Van Helsing, Julia, is attacked by invisible bats. Later she is attacked by real fake bats - it looks like she's being hit with black kites.

Look, this is a bad movie, but if you're into lesbian-vampire movies, you have to watch this. Just on account of the nudity and erotica, it gives _Female Vampire_ a run for its money.