I always start writing at the piano (Kawai MP10) and then notate this into Sibelius, once the structure is in, I start to orchestrate it, then often the piano part is removed (unless the piano has a required part). I only use Sibelius sounds at this stage, I have never found a successful way to get all my VSL library (VSL complete) to work with Sibelius, as the SIbelius template ends up being so huge, it takes forever to load, and that's before you have even written anything !
Once I am reasonably happy with the arrangement, I then create my "orchestra" in VI/VE Pro driven from Cubase networked from another PC. I don't load PRESET or MATRIX, I only build up my own matrices from the patches, this way I only need to load what I am actually using so that the matrices are not unnecessarily large, and it ensures that each of my pieces use what they need, and not what just happened to be in a template. The exception to this, is when I am arranging for a specific orchestra/band when I have to set the template up to match what instruments they have available, and so have to work within that.
Once I have it all ready in VE Pro, I play all the parts in realtime reading from the Sibelius screen into Cubase, then I add Expression maps to cubase and manually select all the articulations. I always use CC for this, I never attempt realtime key switching - it just doesn't work for me.
Once it all sounds good, I record the audio into Cubase track by track from VE Pro/MIR, so that I have individual tracks of audio. A little minimal processing done in Cubase (I don't want to upset what MIR is doing) and that is a finished mix.