By GINIA BELLAFANTE

What happens to MTV veejays when they grow up? What happens when they no longer experience the ineffable rush that comes with introducing bad Aerosmith videos? Unfortunately, many veejays who vow to move on merely vanish into a vast, empty infomercialless ether. Thankfully, Karen Duffy, or DUFF, won't be joining them. mtv's prime-time host-babe, 31 ("senior citizen" in rock-video years), just clinched a four-picture deal with Disney that will include a starring role in an adaptation of The Taming of the Shrew. Says the former model: "I want to play strong-willed wisenheimers."