The big difference between the two is that VSA caches only 2GB of reads from the Master VM1,2. VirtuCache caches reads + writes from all server & desktop VMs, and it can cache to TBs of in-host SSD/RAM, so all storage IO is serviced from in-host cache.
More details in the table below.
EMC Fast Cache Storage Controller Based Caching for VNX Appliances versus VirtuCache Host Side Caching for Any SAN based Appliance
VirtuCache accelerates storage appliances to the same extent regardless of whether they have faster SAS hard drives or slower SATA drives, and regardless of the age and speed of SAN. Thus we help postpone a SAN upgrade, or if the customer is looking at a capacity upgrade, selecting an appliance with cheaper SATA hard drives would suffice. We save them capex dollars either way.