Check out Max/MSP/Jitter
As you describe, the interface is VPL - connecting boxes / nodes to access the GPU is one of the (many) things the program is capable of. Depending on what you're trying to, you may also find Gen
useful for generating GLSL shaders within the Max environment (although you can use other shaders as well).
I'm currently neck-deep in a few Jitter projects using custom shaders, etc., and while it's great for rapid prototyping, getting good frame-rates and production stable code out is a whole black art unto itself. Fortunately, the support and forum community are very strong.