I don't need to create a new site, I only want to add the virtual directories to a pre-existing website. According to the documentation, leaving the IP Address blank and setting the Site Number should allow this.
I have tried this but am running into severe problems.

1: -2151 2: Could not create website
DEBUG: Error 2835: The control ErrorIcon was not found on dialog SetupError
Internal Error 2835. ErrorIcon, SetupError
1: -2147024748 2: CreateVRoot AppCreate
1: Error with IISRT: -2108
DEBUG: Error 2769: Custom Action caCreateVRoots did not close 1 MSIHANDLEs.
Internal Error 2769. caCreateVRoots, 1
Action ended 10:35:28: InstallFinalize. Return value 3.
Has anyone run into this problem, and possible have any workarounds for this?
I'll probably end up rewriting the VDir creation as a vb CA, but it seems a shame to rewrite this functionality if InstallShield already has it available through the IDE.

Thanks,
Jamie