I found running Yosemite (OS X 10.10) in a virtual machine via VMWare Fusion to be unbearably sluggish. If I did not login to the VM within a couple of seconds, the entire OS would freeze. The solution isn't about how 3d acceleration is disabled, but rather involves disabling Beam Sync, which is Apple's v-sync implementation.
Install VMWare Tools, then download and run this build. It is advised to then add it to your Login items, so it boots at login. It will help alleviate the sluggish UI problems and improve performace.
Note the OP mentions this doesn't make Yosemite run as fast as Mavericks can when virtualized. But from my experience, BeamOff has definitely improved Yosemite VM performance enough for basic OS use.