OpenCL: Currently not at all. GPUs are not accessed yet for any of the Hidden Line/Renderworks modes. Eventually I believe yes, but further in the future than 64bit and multithreading most likely. When/if it does, I recommend wearing ear protection because of how loud my singing and dancing will be.
Ram limit: Yes. Vectorworks itself can use ~3.5GB to ~4GB of RAM maximum. Renderworks can take advantage of over 128GB of RAM technically, but currently everything else in the world (HD speed, CPU limits, Software limits) cant even get close to that. If you have 12-24GB of RAM, you most likely have more than enough.
Personally I really would not bother with it. The integrated graphics will "work" but the experience, especially in anything 3D, would be slow and irritating. As of this post, the iMac with Nvidia graphics is the lowest end Apple product I would recommend for using Vectorworks.
When it comes down to price, if Vectorworks is your bread and butter and you work primarily with OS X, then the extra cost of a $1500 iMac compared to a much less powerful $800 Mac Mini is totally worth it.
If Vectorworks is something you do not use often, or is used as more of a hobby, then the Mac Mini would do the job in a cheaper manner, both in price and experience.