One of the Darton crossbows...the Serpert II...comes with Limbsavers. I know very little about Dartons, but if they're all split-limb, Bowjax might make a dampening kit for them. I know they make them for TenPoints - also split limb. They seem to make a significant difference when put on a TenPoint. Typically, things that are attached to the string cost you more speed than things attached to the limbs.
You will not be able to make your crossbow as quiet as a verttical compound, nor will you be able to make it quiet enough that the deer will not hear you shoot before the arrow gets there.....(the second part is just physics)...but any vibration/shock/noise you can take out of the equation is a good thing....even with the loss of a little speed.