First use Vibram's own chart with a measurement of your foot's longest dimension: heel to longest toe.
Then try that size on. Your toes should almost be grazing the ends of the pockets with your heel firmly in the back of the shoe. If you've more than a quarter of an inch (6mm) of space, size down; if your toes are pressing at the ends, size up.
If you have narrow feet, you might get away with a smaller size in some styles. Conversely wider feet might need a larger size in some styles.
The only way to find out your true size is to try them out I'm afraid.