If you installed Windows 7 on the previous Vista partition, then it overwrote Vista and placed Vista's files in windows.old.
In order to have a dual boot with Vista now, you will have to reinstall Vista to a new second partition.
Follow the steps given in tutorial linked above by FZ21Z.
You might also want to save the windows.old C:windows>system32>drivers
file so you can browse there from the Device Manager after Vista reinstall if there are any missing drivers. Other drivers and apps can be found on the Support Downloads webpage for your computer model, the Adobe (flash, reader) and Java (runtime, free Ofc Suite) websites.