You can use your vault to deposit your income if you don't intend using it straight away to safeguard it against future attacks. However, 10% will be deducted from each deposit you make

If your defence is good and you win most of your battles, then I would advise against banking

Many players don't bank at all. Their philosophy is that the amount they loose when someone attacks them is less than the 10% you will be charged to make a deposit into your vault. This holds true as long as your health isn't high for long periods and your defence is strong. The higher your health, the more chance you have of being attacked often and thereby loosing a substantial amount of your hard earned $$