Hello Adam!
The new VSL Soundset Installer (v.131) has been updated for Sibelius 2018.1. It seems it has a glitch for Sibelius 8 now. We will provide a fix as soon as possible. In the meantime, I send you the previous version, which should be working for Sibelius 8. Sorry for the inconvinience.
Best regards,
Andi
Vienna Symphonic Library