All VSL libraries (and every other library I have come across) are non transposed, so all transposition is done within the scoring software. Sibelius and Cubase for example both do this (and I'm sure everything does as well). 

So play everything into the scoring software non transposed,  and use the scoring software to transpose the view (it does not affect the performance). If you are reading a transposed score, then transpose your MIDI keyboard  by 3 semitones to cancel the written transposition.

The only issue I can see using a Bb Clarinet instead of one in A,  is that the A clarinet goes one semitone lower,  so try not to use the bottom Db.

Although there may be an argument that an A clarinet has a different tone, but that's another issue all together.....

When using samples, the only reason to pay attention to keys of instruments is whether or not their timbres are different due to the tuning of the original.  For example, the C trumpet, the Bflat trumpet have slightly different timbres.  The A and Bflat clarinets are very similar in timbre, though when listening to historic recordings you will hear some differences due more to the different instruments' construction than to the tuning. 

VSL recordings are precise enough to actually register these timbral differences but in the case of the clarinets the difference is negligible until you start comparing the Eflat Clarinet with the B flat, or the Bassett Horn with the Bass Clarinet.

I normally let the sequencer do the transposition for all transposing instruments and record the written parts. The tricky bit, are then the keyswitches so depending of the amount of keyswitching required I may transpose the recorded part back to concert pitch.

What you end up doing in reality is playing an A clarinet part using the keying of a Bb clarinet. While this is done in practice, you pretty fast end up in keys that are awkward if not impossible (think ornamentation e.g.) to perform.  (Also one of the reasons why the A and Bb clarinets are still used in pairs today). 

Besides this practical point, each tone on the clarinet has its distinct character (some significant) and the character of the part does change by transposition. I'll bet most will not notice. Also for sampled music you would need a library that has sampled every half note in range to hear a difference.

@Another User said:

Besides this practical point, each tone on the clarinet has its distinct character (some significant) and the character of the part does change by transposition. I'll bet most will not notice. Also for sampled music you would need a library that has sampled every half note in range to hear a difference.

This is also true, but the fact that it is a sample perfomence will rear its head long before you get to worry about slight timbral differences.

DG

@MWE said:

. Also for sampled music you would need a library that has sampled every half note in range to hear a difference.

That's exactly how the VSL library is recorded

@MWE said:

I normally let the sequencer do the transposition for all transposing instruments and record the written parts. The tricky bit, are then the keyswitches so depending of the amount of keyswitching required I may transpose the recorded part back to concert pitch.

What you end up doing in reality is playing an A clarinet part using the keying of a Bb clarinet. While this is done in practice, you pretty fast end up in keys that are awkward if not impossible (think ornamentation e.g.) to perform.  (Also one of the reasons why the A and Bb clarinets are still used in pairs today). 

Besides this practical point, each tone on the clarinet has its distinct character (some significant) and the character of the part does change by transposition. I'll bet most will not notice. Also for sampled music you would need a library that has sampled every half note in range to hear a difference.

edit - I realized I don't understand this question since there is no transposition with sequencing or sampling, only notation, so I should not attempt an answer. 

@MWE said:

What you end up doing in reality is playing an A clarinet part using the keying of a Bb clarinet. While this is done in practice, you pretty fast end up in keys that are awkward if not impossible (think ornamentation e.g.) to perform.  (Also one of the reasons why the A and Bb clarinets are still used in pairs today). 

That is funny you mention that as I am doing a sample performance of that piece - one of my all-time favorites!  It does have a finger-twisting clarinet solo in the middle section.   However, the use of Bflat clarinet works just as well as A with samples.  Though I understand what you are saying how it could be difficult to transpose live.

@William said:

That is funny you mention that as I am doing a sample performance of that piece - one of my all-time favorites!  It does have a finger-twisting clarinet solo in the middle section.   However, the use of Bflat clarinet works just as well as A with samples.  Though I understand what you are saying how it could be difficult to transpose live.

I know that.  It is obviously a fingering issue as I said and the different keys of the instrument affect that. 

However, as stated previously, this has nothing to do with samples.  But it is worthwhile hearing about the actual playing problems.