I only have 8 sata ports on my LSI card that is passed through to unraid and would like to take my current cache drive off the LSI card and use it as a data store.  Could I create a virtual hard disk that is stored on this drive and use the virtual as an unraid cache drive?

You could do that but a Virtual Drive would have slower performance than you get with a real drive.  Don't have a good feel for how much of a performance hit it would be - sorry.  But I use Virtual Drives as boot devices for my Windows VMs and unRAID and it works fast enough for me.

I've been running the setup you want for over a year with no issues.  Virtual disk lays on top of SSD and there is no any perceivable loss of performance. I can tell that because I previously used same SSD, but connected as a physical cache drive in unRAID.

Started using virtual disk for the same reason as you- to save physical ports of a disk controller card.

With SSD you will easily saturate a gig network with either solution.