Hi Rangi
I've had this issue too, and it was a bit of a head scratcher to work out!
when i have this issue, its when ive started a new score with one instrument, then during the writting of the piece ive decided to change the instruments, so for example, i started the piece with Orchestral strings, but changed to Chamber strings as they better represented the sound i was after.
if this is the situation your in, try the following.
add a new stave in Sibelius with the violin 2 section of Dimension Strings ( this is the one you want to work properly)
double check your Ensamble setup, your playback configiration AND the Sibelius mixer to make sure that this NEW instrument is using the correct midi channel etc.
Write ANYTHING in the new instrument stave. (i just do a couple of bars with articulation changes so you can check its changing articulations as needed)
i select ONLY this stave by clicking it so it turns blue (this mutes everything else so you only hear that instrument.
i then play the score and check that everything is ok.
If it is, double click the non working violin 2 stave, so that the blue selector highlights the entire stave from start of the piece till the end
Then in the ribon, click 'home' then in the selection section, click 'System Passage'
you entire stave should now turn purple, now copy this purple selection and paste it into the new stave you created and tested earlier. (the system selection option will copy all your system text data, onto this new stave so you dont have to input it again)
now click this stave again until it turns blue, and play the score and check that the playback is working as expected.
if it is...BOOM...delete the old corrupted stave and everything is good to go.
If its not.....the bad news!....ive only ever been able to rectify this by either copying everything to a brand new score, or inputtin everything in by hand again into the new score as the copy/paste function didnt fix it...although thats only ever happened to me once! thank god!
i don't know what version your on, or what OS, but for me on Mac High Sierra and Sibelius Ultimate 2019.9 its a fix. Its not an issue with VSL as far as i can tell, rather a bug in Sibelius if you change instruments alot as your writing your piece
When you read this post you may think its a massive balls ache to do...but in reality its really rather quick and simpIe once you get your head round it! and I really hope this helps mate.
Kindest Regards
Paul