Now though there is quite a bit of lag between hitting a note on the YRG and the VST playing it.
I assumed it would work the same as any midi keyboard.
Well it should pretty much work the same as a midi keyboard....have you got a keyboard connected that doesn't have the lag??
The latency should depend on your buffer settings and also on any latency inducing plugins in the signal chain.
Try lowering your buffers to 256 for live playing and if using effects on the channel or on the master try Cubase' "Constrain Delay Compensation" button.
i7 firstname.lastname@example.org, 12Gb, W7HP x64, Cubase 7.5.20 x64, Steinberg UR28M, Alphatrack, Wavelab Elements 7.2.1.