I always thought that Kriea brought Nihilus & the Exile together so that the Exile could destroy him and she would partly have her revenge for Sion & Nihilus betraying her. Nihilus is going to be weaker from a story point of view because he is tricked into coming to Telos where there are no force users for him to feed on.
As for Visas, she has a link to him, so I'd assume that if Nihilus drained her he would harm himself. (Spotty I know, but so is the whole force bond thing.)