While ggvis is built on top of vega, which in turn borrows many ideas from d3, it is designed more for data exploration than data presentation. This means that ggvis makes many more assumptions about what you’re trying to do: this allows it to be much more concise, at some cost of generality.
The main difference to vega is that ggvis provides a tree like structure allowing properties and data to be specified once and then inherited by children.
Vega plays a similar role to ggvis that grid does to ggplot2. That means that you shouldn’t have to know anything about vega to use ggvis effectively, and you shouldn’t have to refer to the vega docs to solve common problems. However, some knowledge of how vega works is likely to be necessary when you start doing more complex layouts or when you start pushing the limits of the ggvis DSL.