Depends what kind of rap you listen to as to how its pressed and manufactured, even on the underground.
A lot of traditional East Coast underground acts only release shit on vinyl, for the DJ's to spin with and vinyl junkies to buy.
Gangsta rap is made for the trunk
Major label acts often also get a vinyl pressing for clubs and DJS.
More consumers on a whole buy cds, its is cheaper to manufacture...
Although there is more depth of frequency on vinyl it seems only for the purists.
I have a shit load of old hip hop wax going back to 82, then from bout 95 to now its mostly cds.