It doesn't matter since you have still only one spindle.
But if you get more of them, surely separate file systems accordingly and have virtual disks spread across multiple physical drives.
I think the main limitation is the speed of the disk. I've found virtualisation needs lots of fast disks.
You may even make things slower if you partition the disk as you could end up placing files further away from each other on the physical surface of the disk. I'm not sure how much difference it would make, but wwe used to worry about such things alot in mainframe days! e.g putting the busiest partition in the center of the disk to minimise head movement.
OK, i think it will be the best to make own vmdks for /tmp, swap, /var/log, /var/cache and /data
If there is no overhead for the vmware-server as you said to have more disk devices me feeling says me it maybe less overhead to write in sepearte disk-files instead scheduling wrtie-access on one big.
I want sepearate disks/partitions especially for /var/log to prevent filling the system-disk nearly 100% if something goes wrong in the guest and kill the system, on physical machines that will follow in fragmentation - I dont know if fragmentation matters in a virtual disk, but sure it makes nothing better