hi trompetosaurus. I don't have Cubase 5 yet (I have cubase studio 4) so I'm not familiar with VST expression but I can tell you the manual way of doing it IF I understand you correctly on what you're asking and what you're trying to do.
If what you're trying to do is basically learn how to change articulations/patches in Cubase, for example if you want your violins to play legato instead of detache/staccato, etc..if that's what you're asking then I can explain to you the manual way though it is very fast once you get a hang of it.
Look at your Vienna Instruments screen. That's the screen where you load your VSL patches from the special edition folders. At the top there should be 4 tabs "matrix assign, patch assign, control edit, and perform".
First load whatever patches or matrix that you want, let's just say you want to use violins so load your violins matrix (this loads all the patches/articulations like staccato, legato, detache, sustain, etc etc). Or if you want to do it manually then load one patch at a time like let's say violins legato, then drag the matrix box out to create another box and then load something else like violins staccato etc.
Next click on the CONTROL EDIT tab that I mentioned before at the top.
This tab lets you set up key switches so you can switch the articulations. There are 3 fields you see there "H-span" which means horizontal span, i.e. it's the horizontal line of boxes in your matrix. V-span which is vertical i.e. the vertical line of boxes in your matrix and "A/b Select".
Let's say you loaded a violin staccato patch and then another violin legato patch horizontally next to it in the matrix boxes on the left. We can control these with the h-span key switches. Where it says "not assigned" in the h-span box click "key switch" and a C1, C#1 should appear. This means that whenever you play the low note of c1 on your keyboard it will activate the articulation/patch in the 1st box of your matrix, like violin staccatos if that's what you loaded. Then if you play C#1 it will activate the patch to the right of it horizontally in the matrix, such as violins legato if that's what you loaded in there. It's best to keep this keyswitch at a note that's out of the range of your instrument so if you're using violins then C1 or C2 is good whereas if you're using doublebass it's best to use something like c6 or c7 since the key switch note is supposed to be silent, otherwise your virtual instrument will actually play that note you are using only as a keyswitch.
But you can set it to whatever you want. Now basically in Cubase if you go into the key editor from the top "MIDI" tab, and if you switch to the pen tool, you can just draw a note in the cubase piano roll (key editor) at whatever note corresponds to your key switch in order to make that articulation switch. So let's say you recorded a phrase where your violins play 3 notes in a row like La-La-La...if you draw in a C#1 in the key editor right before the 1st note, when you press play it will automatically switch your articulation to violins legato (or whatever you have in the 2nd horizontal row in your matrix) and then if you draw in a C1 it will switch that articulation back to your 1st one (violins staccato or whatever). You can also just hit record and "play" this note with your midi keyboard into the sequencer so you don't have to 'draw' it with the pen tool if you don't want, it's up to you.
One final thing though is that apparently Cubase and Vienna have two different understandings of the octave system on a keyboard(at least in cubase 4) so a C2 in Vienna Instruments "control edit" will have to be a C1 (1 octave lower) in Cubase. Meaning if you setup your key switch to be C5 and C#5 in Vienna, you have to play C4 and C#4 in Cubase in order for it to make that key switch work.
Let me know if you have any other questions because I'm not sure if I even addressed the right question that you have but I type really fast so I don't mind typing it all out.