The size of your working data set needs to fit into the cache. So specify the size vFRC
The number of buckets depends on the bucketsize and the size of the cache. The bucket size needs as near as possible to match the IOsize of the vDisk. If the bucket is to large you waste portions of your your cache. If the bucket is to small you have increase latency because more buckets are needed to store the IO.
I have show you how to measure the given IO Blocksize of your vDisk.
Why not simply add 10% of your vDisk as vFRC cache and choose 8KB just for start. After a couple of hours fetch the vFRC stats and see the numbers.
Regards
Joerg