both are similar words, but ...
brutal has undertones of the physical damage.
vicious is about intent of the person/thing.
Examples:
The dog attacked viciously. (the dog was mean spirited and meant to attack)
The boy endured a brutal beating. (implies there was a lot of physical damage to the boy, but nothing is mentioned about the reason why)
The woman's reply was viscous. (the verbal attack was meant to offend/hurt the person)