It truely depends on what has happened to your boot files during the initial dual boot and also on how the dual boot was originally set up.

1) If a third party boot manager was used then you must use it to remove the boot options through their own management console.

2) You can edit the Boot .ini files to remove the unused boot options .....you must know exactly where the MBR is stored so as to not remove the wrong options. If the Master Boot Record is stored on a seperate partition from the operating systems then you can run into major issues in doing this!

3) Save all your data to a clean partition and use the vista installation CD to remove all the unneeded partitions and reformat/reinstall the Vista operating system.