You might be interested to see this thread where I raised the same issue...
http://community.vsl.co.at/forums/t/17746.aspx
IMO you're not doing anything wrong. It's my conclusion that VSL's idea of what a legato (slurred) sound is is not the "norm", regardless of how well-intentioned they were with their concept and the programming of these sounds. You shouldn't have to "play sloppy" or expect that the 50ms time threshold is universally suited to work with the character of every possible phrase at any given tempo. (Actually, if I'm not mistaken, the threshold varies from patch to patch. If I'm wrong about that, someone please correct me). On some patches there is excessive "glitching" between half and whole steps! Not only isn't it realistic, but it's certainly not well-suited for producing legato/slurred passages on most instruments where the notes would occur within a single register or position (clarinet, flute, trombone) or string (violin, etc.).
Sometimes the little clarinet and bassoon note-transition "bubbles" or small pitch weirdnesses you'll hear in brass are to be expected when playing difficult passages or wide leaps, particularly when crossing register breaks. Per my training, legato playing implies an avoidance of such breaks or bumps in the tone over the course of a passage. It's hit or miss with VSL legato...
And overlapping notes (or not) will not necessarily solve the problem consistently from one VSL legato patch to another, or even within the same patch on a singular instrument.
My suggestion to you is to either use the reverb approach (where the reverb smoothes the transitions; this commits you to a reverb which may or may not be desirable) or, use other libraries to give you the sound you're looking for. Otherwise you will beat your head against the wall trying to make the majority of VSL legato patches to actually sound legato.
Having said that, I will still occasionally load up VSL legato woodwind patches when my writing calls for legato or slurred passages, just to see if perchance they'll work. Most of the time they don't. Occasionally they will. When they do it's a beautiful thing. But don't beat yourself up if you can't get these patches to sound right all the time. As I see it, they're not programmed to produce the sound you'd ordinarily expect from "legato" out of the box.