Keep in mind that none of the MacBook Pros other than 2011 15"+17" MacBook Pros have 4 "real" cores. They use dual-core CPU with hyperthreading turned on, which gives 2 additional virtual cores and so, gives the apperance of a 4 core. Those virtual cores give only approximately 10~20% of a real core power, so do not assume you have a quad-core if you're running an older MacBook Pro model.
As for CPU assignment, keep it at 1 vCPU unless you have a specific reason to do so. There's no performance benefit or even detrimental if the applications you run don't utilize multi-CPUs, and even if it does, if the OS X is starved in CPU resource, your VM will become sluggish too. If you plan to run multiple VMs, do not exceed more than half of the physical core your mac has it available. OS X needs to have some CPU resource available for its process and to process the VM process as well.
For memory assignment, generally you don't want to assign more than half of the physical RAM you have on your mac. But my experience was that even assigning 50% of your physical RAM to the VM can make the whole machine crawl. So my recommendation is to assign around 25%~40% of your physical RAM. So for 8GB RAM, assign between 2GB~3GB for your VM. If you plan to run multiple VMs, spread out the 25%~40% memory assignment to all the VMs, evenly or give priority to the most important VM.