It sounds like you simply need to make some adjustments in the Sibelius MIXER window. For brass staves, make sure the "device" setting is going to your Keyboard (or wherever you want those sounds to playback from.
For your VDL sounds, make sure the "device" is set to the SB card, and then also double-check the bank high, channel, and program settings to ensure accuracy.
If you haven't downloaded the latest sound-sets from the tapspace support page, these can be very helpful for Sibelius 3 users to automate the assigning of instruments. If you do this, it's important that you're using the "Virtual Drumline soundblaster" soundset, and that you've loaded your VDL soundfont into BANK 001 of your soundblaster card. If so, assigning the instruments in the mixer window should be easily handled by selecting the VDrumline instruments from the "sound" menu in the MIXER window.