Hello Paul!
Don't use standalone Vienna Instruments or Vienna Ensemble with Sibelius. Please use the Vienna Ensemble plugin from your Sibelius Available devices list.
If Vienna Ensemble doesn't show up by itself after activating it, you can make it appear by clicking on the active device Vienna Ensemble in Sibelius, then "Show" and then in the Vienna Ensemble Service Interface on "Raise".
Best,
Andi
Vienna Symphonic Library