In South India, most single composition performances have words - with improvisation done in sol-fa. If an alaap/taanam (jor) is performed, meaningless words are used. In the north, Dhrupad vocalists take the same approach - but rarely use sol-fa passages. Instead the words of the composition are used for improvisation as well. In Khyal, the composition is just an excuse to convey the music. The words of the composition ends (or consist of)with vowels, which the artist stretches to convey the notes...so lots of aaaaaa. In thumri words are the primary medium to communicate the melody.
As Lars indicated...visit http://www.sawf.org/music/articles.asp?pn=Music and click on any of Parrikar's articles.
My tabla techer was a singer by profession, and i picked up some stuff, she used a lot of Sa Re Ga etc.. for practise, but when improvisin a lot of Aaaaaaaa... but also the Sa, Re, i think it deppends pretty much on the type of song and performance.Also words. listen to Ustad Nustrat Fateh Ali Khan, hes not stricly north indian but from pakistan Qawalli music, they have words but when he improvises he moves to syllables.. amazing guy..