The most obvious is garlic. Garlic is thought to ward off vampires because they cannot tolerate the smell of it. It is said that it can even be hung around doors and windows to keep them from entering your house. Some legend also states that if a vampire consumes the blood of a person who has a lot of garlic in their blood then they will become very sick or even die.
Verbena is a flowering plant and is also thought to ward off vampires because of its fragrant qualities. Folklore says that it can ward off other evil creatures as well.
Poppy is another plant that is thought to have the same effect as verbena. They are both thought to have many magical qualities.
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.
Although it is not herb-related, Bram Stoker is the man responsible for some of the ways we see vampires today. For example, it is his novel (not folklore) that first stated a vampire cannot see his own reflection. The idea that people lacking souls had no reflection was folklore, but whether or not Stoker knew this or not is unclear.
Garlic, stakes, crucifixes, and garden-variety herbs have been used for centuries to ward off "vampires." The reason man started using headstones in cemeteries was to keep the dead from rising out of their graves.
The rumors of vampires most likely began because, years ago, people did not embalmb their dead; thus, people literally were buried alive - causing their bodies to look fresh upon opening the caskets. Patients in deep comas were often buried, and scratch-marks were found upon opening the casket lids. For a long time, people believed these poor folks were vampires. A bit later, a string with a bell attached would be buried with a dead person, just in case they were buried alive. This is where the term "saved by the bell" originates from.
I hope this helps you. :)