Hello Eric!

I have seen in our records that you have registered Solo Strings I + II full.
The glissando line will trigger portamento by default, just as the portamento line. Portamento is a fast slide to the target note. So to finish the transition in time, a value of -10 for the live start position should be about fine. If you are using Vienna Instruments Pro and you want to avoid that the note overlaps with the previous note, you either have to shorten the previous note or you have to use the Mono Interval mode (Advanced/Options).

I have seen in your example that you have used a slur between the first notes, although you probably mean a tie. Your example will sound better with a tie. The portamento line between bar 5 and 6 in your example doesn't reach the target note. That's why it has no effect. It may be necessary to draw the line further the note. You can see, if the line is long enough, when you click the target note and the articulation in the Vienna Instruments (Pro) player changes to portamento.

In order to trigger glissando, you will have to work with custom cells as decribed in the "Optimizing Sibelius Playback" manual, page 12.
http://www.vsl.co.at/en/MyDownloads/Notation_Related

Best regards,
Andi