If VSL was really to do something about the legato in its current form, I believe re-editing or possibly re-recording would have to be done. I'm not an expert when it comes to editing or scripting by no means, but to my ears it sounds as if they cut into the transition samples quite aggresively, compared to legato patches of some other companies' libraries, and the sustain part is kind of tacked onto that, and the attack is very immediate.
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The manual shows 6 legato types:
soft
normal
marcato
fast
fast marcato
slur
!!!
marcato is just combination of legato and marcato articulations