There are others in the ITProTV membership that probably have more experience on virtualization than I do and I hope they chime in!
Usually, the rule of thumb for vCPUs on a VM is to use them only if you have applications can used a multiple CPUs. Also, if you only have a Quad-Core, do not assign it more than that (no 8 vCPU, just because you see you can do it.)
Now on the other side of that coin is that you will experience performance issues too that is because of CPU scheduling. I do know of some admins that will say every piece of software is expecting to see 2 cores so you should do it but I would check the min requirements before doing so.
**if the post above has answered the question, please mark the topic as solved.