***Unknown Hammer vampire flick is subpar, but has its attractions*** Early in the 19th century a dying vampire curses a Serbian village, which is visited fifteen years later by a sinister circus of vampires & their acolytes who seek to resurrect the diabolical Count via the blood of youths from the hamlet. “Vampire Circus” (1971-1972) is an obscure Hammer production from their declining years. It’s technically not as good as the contemporaneous “Countess Dracula” or the earlier “The Vampire Lovers” (1970), although it arguably contains more sensationalist thrills. Along with the slightly…