Vista does not create a new partition when reinstalling over an existing instalation.
What it does do is collect the previous instalation into one folder called Windows.old.
In order for it to create a new partition, first you would have had to have suffcient unalocated free space on the drive. On starting the instalation, you would have been asked where to install Vista and had to select the unalocated space to create a new partition in.
Maybe you can take a screen shot of your drive management screen showing the drive partitions so we can see what you have.