Hi Anthony. It's all in the fingering - to trigger the legato effect every time, every successive note in a melody line has to overlap with its predecessor. It's a sloppy way of playing, but unless you do it the VI 'thinks' you want a detaché effect and doesn't output a legato interval. It's unnatural to play that way and I often end up with missing legato notes in a sequence - I get round the problem by opening up my sequencer's graphic editor and making sure that I can actually see all the notes overlapling.
There are certain limitations with VSL legatos:
1. The effect is monophonic, i.e. only works on single melody lines. To create true legato chord movements you'd have to build the chords up out of individual lines.
2. The legato intervals were only sampled up to an octave above and below the starting note, so if you play a tenth interval you won't hear a real legato.