You will get better performance with 1 vCPU. I ran Zimbra on VMware Server 1.x on Ubuntu 8.04 host and guest with two 160GB SATA drives in Linux RAID1 with 5 other VMs on the machine (two Windows XP, Windows 2003 DC, and two Ubuntu). Zimbra worked great after I disabled the logger. I saw major performance issues when logger was enabled because of how many disk IOs it uses. Disk IO seems to always be the performance issue with virtual machines. My box only had 6GB of RAM, 2.33 GHz Quad-Core XEON, and the Zimbra VM had 1 vCPU with 1.5GB of RAM. The RAM was preallocated and no swapping of memory. There are a lot of tuning you need to do on the VMware/host side to have good performance on your virtual machines.
PS: I used Ubuntu 8.04 JeOS (32-bit) as the guest OS.