"In a world governed by magic, you may find the Law of Similarity, valid."
What's the Law of Similarity?" asked Bayard sharply.
"The Law of Similarity may be stated thus: Effects resemble causes. For example, you can make it rain by pouring water on the ground, with the appropriate mumbo-jumbo."
"Another is the Law of Contagion: Things, once in contact, continue to interact from a distance after separation."
Chalmers continued. "From the elementary principles of Similarity and Contagion, we now proceed to the more practical applications of magic. First, the composition of spells. The normal spell consists of several components, which may be termed the verbal, somatic and material. In the verbal section, the consideration is whether the spell is to be based on the materials at hand, or upon the invocation of a higher authority. And the verbal component should conform to the poetic conventions of the environment."
However, there is also a somatic component to a spell, subject to more precise regulation. There is some point in connection with this component that eludes me."
(The Compleat Enchanter, L. Sprague deCamp and Fletcher Pratt)
While the old-school D&D magic system is often described as Vancian, it is actually a mish-mash of different magic systems, mixed together in an unholy goulash. Take the magic system described in deCamp and Pratt's magical misadventures of Harold Shea. In that system, spells can be created, without advance study or meditation, as long as appropriate material, verbal and somatic components are combined. While the impressing of spells upon ones mind was borrowed from Vance, the idea of material, verbal and somatic components, central to the magic system in AD&D, comes from deCamp and Pratt.
While I have a deep attachment to Vancian magic's daily spell preparation (from a game mechanic standpoint), I like the freedom of the Prattian magic system: players can produce any spell effect, on the fly, as long as they can come up with a reasonable argument for particular spell components, and can utter a convincing rhyme that would invoke the spell.
Hell, i'd give out experience points for that kind of "player skill" spell-casting.