vocalise is a vocal work or an exercise without words.
As an exercise, it is often used to develop flexibility
and control of 
pitch and tone.
Years back I studied conducting with Emanuel Balaban who was on the Julliard faculty; heady stuff.

He was "old school" meaning one of your conducting skills was that you had to be able to sing in solfege to conduct and that singing was with syllables; do re mi...  do do (pronounced dough) sol sol la la sol, fa fa mi mi re re do  is twinkle twinkle little star - try it, you'll like it. If you want to get more technical, then remember the Sound of Music song ... "Do a deer a female deer....

All his students studied out of the Danhauser books (1,2 &3) Solfège des Solfèges and it got hard very fast.  I wasn't very good at it to tell the truth; actually I'm much better now singing while driving the car....slow learner.  The real treat was being "allowed" to sing on a neutral vowel, "ah" for instance. Then you could bellow in your most operatic shower voice...

I got to do this piece by Rachmaninoff and with Balaban accompanying me on a huge Steinway grand in his apartment on W. 71st....well.   I still have the music that we used - my part anyway.