Slick's images, polygons, lines, circles, etc. are all rendered using the GPU unless the user has old/broken drivers or they have somehow disabled hardware acceleration (in which case they probably shouldn't expect any OpenGL games to work).
There are a variety of things in Slick, like polygon triangulation, transform/scaling/rotation, etc that are done on the CPU.
What kind of vector calculations are you doing that need to take advantage of the GPU? Even AAA titles generally don't need to take advantage of the GPU for math/physics -- PhysX has supported GPU physics for a while, yet the list of games using it is very short
Generally speaking the CPU will be more than fast enough to handle all of the physics and math in your game -- if you are finding it a bottleneck, I'd wonder whether your algorithm is slow, rather than the processor. For a 2D game, especially, the CPU should be suitable.
Calculating physics on the GPU is not a small task -- OpenCL is pretty new and finding documentation/tutorials on using it may be challenging. It's also not very widely supported yet, and many of your users will be unable to play your game.
If you really
need hardware accelerated calculations, you can use GLSL (shaders) to achieve this. Here's a brief intro on shaders in Slick:http://slick.cokeandcode.com/wiki/doku.php?id=shaders
Chances are, whatever you're trying to do should be possible with plain old CPU calculations -- or GLSL if you have more specific performance needs.