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.
How to Select SSDs for Host Side Caching for VMware – Interface, Model, Size, Source and Raid Level ?
Dell's PowerEdge VRTX hyper-converged appliance can either have all hard drive datastores or all SSD datastores, but you can't have SSDs act as tiering or caching media for HDD volumes. That's where VirtuCache comes in.