When you create an answer file for unattended installation of Vista, you must specify which partition and which disk drive you want to install the operating system onto. You normally do this by configuring the following component:
Normally, you configure the settings named DiskID and PartitionID for this component. But if you simply want to install Vista on the first partition of your first hard drive, leave these settings blank and instead go to the following component:
and set the value of InstallToAvailablePartition to True.
For more information on deploying Vista, see my series of articles titled Deploying Vista right here on WindowsNetworking.com.