You'll need to add a (or replace the) winpeshl.ini to launch a script of some sort. It can be .cmd or something else you make. This script needs to be able to determine the drive letter that the DVD resides in.
Create 2 answer files. The first one is your Unattended one. The second one should be blank but with the correct XML elements so that Setup can parse it. Name it Attended.xml.
Your script will have 2 options:
1. Unattended
2. Attended
Presuming your DVD drive is D:, then the two option above would have the following commands:
1. Unattended
setup.exe /unattend:d:\autounattend.xml
2. Attended
setup.exe /unattend:d:\attended.xml
There does not appear to be a command line parameter to instruct Setup.exe to NOT look for an answer file. This is why you would need to use a relatively blank one. Here is an example (not tested):
<?xml version="1.0" encoding="utf-8"?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">
<settings pass="windowsPE">
<component name="Microsoft-Windows-International-Core-WinPE" processorArchitecture="x86" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<SetupUILanguage>
<UILanguage>en-us</UILanguage>
</SetupUILanguage>
<InputLocale>0409:00000409</InputLocale>
<SystemLocale>en-us</SystemLocale>
<UILanguage>en-us</UILanguage>
<UserLocale>en-US</UserLocale>
</component>
</settings>
<cpi:offlineImage cpi:source="catalog:d:/sources/install_windows vista business.clg" xmlns:cpi="urn:schemas-microsoft-com:cpi" />
</unattend>