This is weird. One thing I will give Vista credit for is how well it works with dual monitors. For my laptop, it automatically detects it when I plug another monitor in and gives me the option to use clone or dual view...
As someone said, confirm 100% you have your drivers loaded. Not just Microsoft drivers, but the manufacturer's drivers. Also look on your keyboard. Sometimes there are mode buttons (function F4 for me) that will change the view mode.