The first and most obvious answer is the history of garlic as a means of repelling vampires. This legend traces its history to ancient Romania. The Romanians believed that garlic possessed the ability to ward off evil spirits of all kinds. As such, its use against vampires, seen as representative of evil, was a natural choice. For more on that specific lore, see http://www.garlic-cloves.com/garlic_and_vampires.html
Other herbal remedies included Hawthorne and Rowan (Ash) and several other woods for use in the stake that is driven through the vampire's heart:
In many of these legends, the stake must be of a
particular wood, such as ash, hawthorne, maple, blackthorn, buckthorn,
or aspen. The power of these woods is often claimed to lie in Christian
symbolism. For example, ash has been cited as the wood from which
Christ's cross was made. Plants with thorns, such as hawthorn, wild
roses, and blackthorn, are associated with the crown of thorns worn by
Christ at his crucifixion (Kuehl, nd).
As less successful but nonetheless used in literature method is the scattering of seeds in the path of the vampire under the theory that he or she will become so caught up in counting the seeds that he or she will forget about coming after the victim. This method has not been put to all that much use, however, and it has not been effective in much vampire literature and lore.
Although not an "herb" or plant, holy water is another method of choice against the undead. Again, this relates to the fact that they are evil and something good, like holy water, is toxic to them.