No, it's not that they will both drain the life out of you.
It's not even that the undead have a lot of time to read and therefore learn a lot of words.
With enough time, we could all gain better vocabularies than Shakespeare who, at around 30,000 words*, is often thought to be at the apex of such things. Most of us blunder about with only 10-20,000 words at our command. Having said that though, Shakespeare was not the most eclectic word user of the great writers. Marlowe is thought to have had a greater number at his disposal, and certainly used more different words per play than Shakespeare, as did Jonson. Some assert that Shakespeare typically tops the list simply because he wrote more plays. Time, it seems, carries great lexicographical weight. One could expect a timeless bloodsucker to know more words than anyone else.
Still, enormous numbers of words are not the connection between vampires and grammar. That was, I'm afraid, a blood-red herring.
The connection is far more interesting than that. Not that vampires aren't already interesting enough. All those questions of life without life, the meaning of death, the barely (or not at all) concealed sexual energy and symbolism threading through the tales of lustful fangs and dripping necks. It's enough to make you want to sit in front of the tv and binge-watch True Blood (come on, admit it, you already have).
True Blood gives us a clue to the connection. When Vampires on the show use their supernatural powers to render their victim open to persuasion, and thus convince them to open the door or other such plot-turning devices, they are said to "Glamor" their victim. (Or "glamour" as it is spelled everywhere except America.)
Glamour is a word people associate with celebrity and fashion. A star is glamorous on the red carpet. It suggests a certain allure; a captivating appeal.
Aha! So someone who is somehow "otherworldly" to us, by virtue of their fame, or money, or status, has the ability to magically gain and hold our attention. And then they persuade us to buy things like watches, cars and shampoo. So they're a bit like True Blood's vampires, taking us into a trance, making us suggestible, and then controlling our behaviour. Celebrities huh!
This sense of glamour is actually as old as the word itself. Because glamour is actually a Scottish variant of the word grammar that dates back to the early 18th century (as a noun, and then we see 100 years later in 1814 the first verb usage as seen now in True Blood). The magical connotation stems from the fact that the word in its Latin and French roots implied education in language as well as spells and incantations. The occult and language were close bedfellows and a person educated in grammar knew both.
So it was that through the 15th and 16th centuries and beyond, to be highly educated in the arts and letters was thought, essentially, to give someone the ability to enchant the less erudite. There was a certain magical aura to the highly educated, just as there was a magic surrounding those of high social status, such as lords and kings. Little wonder then that to have grammar had a certain allure. Indeed grammar slots in nicely with words like alluring, enchanting, spellbinding and captivating.
So, what do vampires and grammar have in common? Well, it turns out that vampires are masters of grammar. It probably explains the pale complexion of your primary school English teacher. Quickens the pulse, it does. Better bring the garlic to your next class...
*Estimates vary from the mid 20 to high 60 thousands. Which is as much to say we don't know how many words he actually knew, we just know how many are in his plays. Even then, the count is muddled by whether one should count each variant of the same word or not. If you don't, Willy comes down to normal human numbers. Myth busted folks. See more here: http://www.dispositio.net/archives/501