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The Vector tool allows you to easily create smoothed shapes filled with a solid color or gradient.
It's perfect for working on cartoons, caricatures, as well as quickly blocking in shapes for use in just about any style!
Unihorse has created a couple wonderful tutorials that show how to use the Vector tool.
Vector tool and backgrounds 
Two layer vector tutorial Check out some other great examples of the Vector tool in action!
Sketch Wars by Lescot 
Oh, the Thumbanity!!! by Iggycrypt Vector tool gradients can be combined with the Smudge tool to produce amazing clouds and fluffy things. In this video the amazing Roberto Rendón demonstrates his technique for clouds using Vector and Smudge.