I think the image that Justin Tokke showed doesn't mean that a note is to be played an octave lower.
That notation means "coll ottava", which means "with the octave/at the octave", basically meaning you should play that note doubled by the note an octave lower (in this case). So, in the passage shown, you should play the A written WITH the A below it. Not just the A below it.
If you want to notate a note and indicate it's to be played an octave below, then:
A Million Notation Questions! (read the posts below that as well).
I haven't EVER seen 15vb, firstly because the term itself is wrong (for two octaves, you use 15
ma, which comes from the Italian term "quindicesima", which means "at the 15th", or in practical terms, "two octaves above". If you said "15vb", that would be equal to saying "quindicesava bassa", which doesn't really exist (a mix of "quindicesima" and "ottava bassa" - I guess if it meant something, it would be "two octaves above the octave below" :S ). So it is important to understand how abbreviations and terms came to be what they are, to avoid mistakes like this.
In any case, I haven't seen any kind of "2 octaves below" sign in any piece I've seen so far, not even in Stockhausen's Klavierstucke or Xenakis' "Evryali". There is not practical application for a sign like this.
Also, I am not using sibelius, so I wouldn't be able to help you with the technical stuff, but one solution would be to momentarily (for the bar, or for the length of the sustained notes) write the music in three staves, and use an 8vb sign on the lower stave while writing the notes normally on the other staves.
Writing for three staves has been done (among others) by composers such as Schumann (his "
Romance No.2 in F#", although he does it to make the melodic lines clearer), Messiaen (in his "
Quatre études de rythme", for example, where he had different dynamics and different octaves for each stave), Takemitsu (his "
Romance", where at points he has three staves with different phrases at different octave dispositions) and Schoenberg (in his "
2 Stücke" - where he does
exactly what you're looking for: he has two chords, in very high and very low registers which are to be sustained, and then he has playing in the middle registers).
Hope I helped!
EDIT: Oops! Sorry guys, I didn't see you had already answered that
My fault..