While wearing the Ring of Vigour and using any weapon's Special Attack, you will save 10% of your Special Attack. So if you would normally use 50% of your Special Attack, you'll only use 45%. The Ring of Vigour also gives a +3 Strength bonus, which is +1 less than a non-imbued Berserker Ring.
This item is commonly used by rushers in the Wilderness with a Korasi's Sword. This allows a rusher to get either 3 series of very quick attacks or 2 Magic attacks that are very likely to get a kill if the target is not using protection Prayers or paying attention.