The weak points include: 1) Uneven acting. Lyn Peters is dreadful as Anne. She truly makes Kristen Stewart look like an Academy Award Actress which we all know is not going to happen in this lifetime. The other female leads give O.K. performances but no one is outstanding. 2) It has a few "hippy" excesses such as a little free love scenes and a go-go dance party which seems out of place in this film.
Overall, Grave of the Vampire is decent but I can't get too excited about it. The 1970's wasn't a great era for vampire films. Thankfully, the 80's would give us a wonderful resurgence of the genre.
For more info check out the film's entry in IMDB.