I am currently using Sibelius 5 with VDL 2.5 on a Windows Vista.

If I export a piece of music that I have written and happens to have VDL instruments, when I play it on my computer, only the VDL instrument parts are heard. The rest of the music is just not there, only the VDL percussive parts. It's quiet aggrivating.

What other instruments are you using? They have to be virtual instruments (not general MIDI) to be included in an audio export.  Also, make sure they aren't muted in the mixer.

I'm just using the normal instruments that Sibelius has. Like the Strings and Brass. I don't know if those are Virtual or not?
And i just checked and it wasn't muted.

In your Playback Configuration, do you have an instance of Kontakt Player 2 loaded with ";Sibelius Essentials"; or ";Essentials"; selected as your soundset?  If not, what do you have as your Active Devices?  Fixing that problem should get you up and running.

Yep that did the trick. Many thanks Mr. Harmsen

Ok wait hold up. I did that and the exporting works but some instrument parts just dont play in my music. So far ive noticed recorder and saxophones dont playback. Is there a reason?

If you have a legal copy of Sibelius, have the Essentials library installed, have two instances of Kontakt Player 2 loaded (one with the VDL soundset and one for the Essentials sounset) you shouldn't be having problems.  If you have more than 16 sounds loaded on one of those instances, you need to load another one, since in plugin mode you can only have 16 sounds per instance. 

It doesn't seem to make much sense that just a recorder and saxophone wouldn't end up in your recording.  Are you sure they are loading and playing back in your score prior to exporting audio?  If the instruments are loaded properly, they shoudl show up in KP2 and in the mixer as is illustrated in the attached screenshots.

It might be worth checking out the Sibelius forums as well, since this problem isn't specific to VDL.

Alright I found the problem, but still have one. The reason it isnt playing back is because the original instrument I had in my music doesnt have the same title as what is in the Sibelius Sound Essentials. For instance, I had a Solo French Horn part that wasnt playing back but then I went to the mixer and changed it from normal french horn the solo french horn and it worked. The reason the recorder part is not working is because the original in my music is a Soprano recorder. The problem is that when i go to change the instrument to a normal recorder it doesnt come up as one of my instruments and a soprano recorder is not in the Sibelius Sound Essentials.

Have you reported this on the Sibelius forums?  While we'll be happy to help, none of these problems are related to VDL. 

http://www.sibelius.com/helpcenter/index.html

In the meantime, check out this post as well:
https://www.tapspace.com/forums/index.php?topic=2554.msg13636#msg13636