Hi Stéphane - as you say, VSL's legato and portamento patches are designed for monophonic use, all other articulations are polyphonic. Some sample players let you limit the maximum number of voices down to one stereo voice so that polyphonic patches play monophonically, but I can't see a way of doing that in Vienna Instruments.
However, you can make any VSL patch instrument 100% monophonic by carefully removing all note overlaps in your sequencer after you've recorded a part (Logic has a 'Remove Overlaps' feature which will do this automatically).