One could think of this vibrational essence as being electrical in nature. The water's molecular structure is charged and imprinted with the electrical pattern of the flowers' vibrational essence.
The theory behind the use of the Bach Flower Essences is each of the flowers embodies a particular soul quality (or energy frequency). When an individual resists that quality an energy blockage forms so that s/he is out of touch with that aspect of his/her fundamental nature.
In Dr. Bach's own words:
The action of these Essences is to raise our vibrations and open up our channels for the reception of the Spiritual Self, to flood our natures with the particular virtue we need, and wash out from us the fault that is causing the harm.
It's kind of a spiritual thing, if you could think of a flower having some spiritual attributes. But it is based on the idea that the human being has a spirit body inside of his physical body that gets sick too, and this is reflected in the physical.
This is what is known as "phychosomatic" disease. You are probably more familiar with this terminology. If people really knew what the meaning of this was, they would label it as New Age or occult or something stupid.
But the word is made up of phycho = mind and soma = paraspirit or spirit body. There are so many diseases that doctors just can't explain and so many alternative medicines are able to cure without the aid of conventional medicine.
Why? Because a great number of diseases begin in the mind and spirit. So if you only treat the physical body, they are only treating the symptoms many times and not the source of the disease. Holistic medicine has the aim of treating not only the physical but the mental and spiritual also -- the whole man. Conventional medicine doesn't believe man has a spirit as a rule, so it doesn't consider the rest.