Differences between adding virtual CPU cores and virtua CPUs

I have a virtual Windows 2012 server running on an ESXi 5.5 server.  It was initially created with 1 vCPU with 1 core. Its primary function is to run backups using Backup Exec 15.  Now I want to try using deduplication with my backups.  The BE15 documentation states that for up to 5TB of data (which is a little more than what I am backing up during a full backup), the BE server should have 4 cores and 8GB of memory. I'm a little hazy on the distinction in VMs between cores and vCPUs in terms of performance and/or affect on other VMs on the same host.  Should I add 3 cores to the existing vCPU, or should I add 3 additional vCPUs?  The host machine has 2 processors with 4 cores per processor.

Except for storage requirements, this is a small network (about 50-60 users).  There are 2 hosts and a total of 8 virtual machines. Besides the backup server, there are 3 other VMs on this host, but none of them have high-intensity I/O requirements. One is a DC, one is a web server and one is a file server that stores audio, video and graphics files that are mostly used as a digital archive for the contents of the building (a museum) and also for on-line research.