Vst instrument

I believe you have to use jack to send the midi instructions to another synth to render using vst, although I've never tired this, but you might want to try: https://musescore.org/en/node/37806

MuseScore is, at least, plug-and-play with SoundFonts, and MuseScore's own built-in sounds are to many people more realistic than Sibelius's. I would suggest trying it out—simply export a MusicXML file from Sibelius, open it with MuseScore and press play. Also, no reason to hesitate before downloading MuseScore—a free and open source piece of software gives a whole new meaning to the phrase "no-risk trial."

Haha, more realistic? Kontakt is one of the best plugins to be made, you can use it with sib or finale. Musescore's sounds are some of the worst, especially when it comes to micro tuning.