Framebuffer objects in Cinder (by default) have no anti-aliasing which leaves your edges jagged. The solution is to create a Framebuffer format and specify the number of samples to use. More info on FBO’s and anti-aliasing
MAKE SURE YOU ARE ACTIVELY USING AND REFERENCING THE ATTRIBUTES IN THE SHADER!
Here’s an example of using dynamic custom attributes in GLSL 1.2 and Cinder. I also found this link useful!