While it seems entirely possible that Lovecraft was influenced by the Voynich Manuscript in devising the Necronomicon, there seems no particular reason to suppose he would have needed it for inspiration. The idea of The Necronomicon drew on an already well-established tradition in horror literature that there exists somewhere a forbidden and incredibly dangerous occult book. This tradition, in turn, draws on the Medieval tradition of the grimoire–various real-life texts such as The Key of Solomon which purported to give the inside dope on summoning demons and the like.
Lovecraft wrote a short history of The Necronomicon in which he suggested that Robert W. Chambers, who wrote several horror stories in the 1890s dealing with a dread volume called The Yellow Book, was inspired by The Necronomicon. This was an amusing piece of circular logic, and there seems no reason to suppose that Lovecraft did not have his tongue firmly in cheek when he wrote this. In fact, The Yellow Book was likely one of the literary devices which inspired Lovecraft to invent The Necronomicon.
Interestingly, The Yellow Book was not a grimoire, but a play, one about things so disurbing and subversive that it was known to drive people who read it mad. Because of its dangerous nature, Chambers could, of course, only quote very brief snippets from it. It was apparently about some sort of fairy tale kingdom called Carcosa, and a dread character called The King in Yellow.
There was already a well-established practice among horror story writers of borrowing from and alluding to one another before Lovecraft started the so-called “Chthulhu Mythos” to which Robert Bloch, Robert E. Howard and others (including Stephen King) have made contributions over the years. For instance, Chambrs evidently got the name “Carcosa” and several others he used from the writings of Ambrose Bierce. In at least one story Bierce had a character who is something of an expert on occult lore cite a nonexistent occult book as authority for a claim.
Happily, nobody yet in this thread has claimed that The Necronomicon is real. One can find any number of sites on the Web which claim this, sometimes with apparent sincerity. One expects a fair number of these are people who think The Blair Witch Project was an actual documentary.
There is an old classic cartoon which shows a group of cavemen. One is sitting on the shoulders of his companion, and a third caveman, in turn, is sitting on his shoulders. This third caveman is painting on the ceiling of their cave. The caption is something such as: “This way they’ll think we were giants, or else we had invented the ladder”.
From time-to-time people really do make or do something just to be enigmatic and yank the chains of whoever is going to stumble on it in the future. Hence, perhaps, The Piltdown Man hoax, and the numerous instances in the history of archeology in which anomalous objects have been found, such as Chinese porcelain in Ireland or a Roman coin in a dig at an ancient Native American site.
It does not seem inconceivable that The Voynich Manuscript could be something similar: a purposely nonsensical book created by an eccentric who was amused by the idea that it would confound people in the future. I don’t know if it is impossible that it wasn’t produced with the idea that it would confound people in the creator’s own time; is it conceivable that a hoaxer could have tried to sell an authentic manuscript from the land of the elves?