Lets not do 1.00001 + 1.00001 = 2 engineering please...That doesn't work.
The vibration is not directly related to the displacement, but rather the amount of stroke.
So the less stroke you have, the less vibration there will be.
There are other ways to lessen the vibrations with crankshaft design but it is always a trade-off with something else like performance.
For all practical purposes balance shafts are out of the question unless you have your own machine shop and can live with the extra weight.
Also the two banks of cylinders in a 90 degree V8 cancel out the vibration somewhat (I believe to the square root of 2) to about 140-150% of a standard 4 cylinder's vibration with half of the displacement.
40%-50% more vibration isn't bad to live with. With some nice fluid engine mounts that can be significantly reduced. Personally I'd want to solid mount the motor or use minimalistic thin rubber isolators if I were going to the trouble of making the engine actually perform rather than just sound cool.
OTM