Etym: "Madam" Helena Petrovna Blavatsky was one of the most famous spiritualists, and the founder of modern theosophy.
vampires: Essentially, semi-human creatures who drink blood; the details of the legend have gone through many variations over the centuries. Rowling's vampires retain the aversion to garlic, though the exact mechanism by which it wards them off is yet to be explained; they are also recognizable in part by being very pale and gaunt. Modern Dracula-inspired mythology insists on a kinship with bats and a predilection for wearing black. Okay, kids, now keep a sharp lookout!
Treatment of vampires in wizard society is covered by the Guidelines for the Treatment of Non-Wizard Part-Humans; nevertheless, there seems to be popular opinion in favor of exterminating them anyway.
Vanishing Spell: Might be something to render a person invisible or unnoticed, or another term for Apparation.
Vauxhall Road: A major thoroughfare in London. Tom Riddle bought a diary there.
Vector: Last name of the witch who teaches Arithmancy at Hogwarts.
Etym: From the mathematical term.
veela: Usually spelled vila, these originate from Balkan legends. Veela are the spirits of young women who died before marriage, who appear human except for having goats' hooves. They are said to be jealous and capricious, but not entirely unkind.
Venemous Tentacula: A spiky, dark red plant that has teeth.
Veritaserum: A truth potion. Three drops of it will cause the drinker to answer all questions fully and without falsehood. Testimony gathered in this way is considered unreliable, however, for reasons as yet unexplained.
Etym: From Latin veritas "truth" + serum "serum, whey".
Vernon Dursley: Etym: Alternative for vernal in 1658, or a surname derived from a common placename in France.
Veronica Smethley: Etym: Corrupted form of Latin vera icona, "true image". St. Veronica is a legendary figure who is said to have wiped perspiration from Jesus's face as he carried his cross; the cloth used retained an image. Also the name of a plant genus.
Viktor Krum: Etym: Cognate to Victor, as you suspected.
Vincent Crabbe: Etym: From a derivative of Latin vincens, "conquering". The name of a 3rd century martyr, and St. Vincent de Paul (1580-1660), remembered for his clinics and works of charity. A bit closer to the character we're talking about, also used in 1592 to mean the dupe in a betting game.
Vindictus Viridian: Etym: From the same root as vindictive and meaning pretty much the same thing.
Violet (GoF ch. 17): A friend of the Fat Lady who lives in a portrait in a room off the Great Hall.
Viridian, Vindictus (PS ch. 5): Author of Curses and Countercurses.
Etym: Strong green; technically, Veronese green. In this case, the green of jealousy or envy.
Voldemort: The Dark Lord; He Who Must Not Be Named. His rule was cut short by a curse that rebounded on him when he tried to kill the infant Harry and left him a shapeless wraith. Now reconstituted with the help of Peter Pettigrew, he has regathered the Death Eaters and started in on some serious villainy. Wand: 13 1/2", yew and phoenix feather.
Etym: Probably derived from French vol-de-mort, "flight of death". Beastly equation for "L*rd V*ld*m*rt": a=30, b=33, etc.
Volkov (GoF ch. 8): A Beater on the Bulgarian national Quidditch team.
Etym: From Polish wilk, "wolf", probably from an Old Slavic given name or nickname.
Voyages with Vampires: One of Gilderoy Lockhart's books.
Vulchanov (GoF ch. 8): A Beater on the Bulgarian national Quidditch team.
Etym: No name info. Might be from something like volchonok, Russian for "young wolf", to go with Volkov.