Readyboost likely won't help much in this particular situation. It's a caching mechanism of sorts, where Windows watches the I/O operations on the disk. Over time if it identifies access patterns better suited off a flash storage, it will migrate a copy there. Things like the windows boot process are accelerated, due to the flash drive allowing quicker random reads compared to a spinning disk.
As far as Windows is concerned, I/O access to your VM hard disk images is pretty random, and likely difficult to cache. It has no visibility into the files inside the virtual hard disks, so it can't tell the difference between the VM reading a specify file, or reading the file allocation table. If the VM hard disks are one solid file, it may never cache it, due to size. If it is set up as a sparse VM disk, then some pieces may make it into the cache if the chunks are small enough.
I will say it shouldn't hurt peformance either. If you already have a usable flash drive, give it a shot and see if it provides a noticeable benefit.