Vicuclad bricks can break for the following reasons: the brick is hit by a log that is at a high temperature; logs are not placed, but thrown into the firebox chamber, age, overheating,or moisture ingression which weakens the vicuclad board. If logs are placed with care into the stove vicuclad firebricks can last for many years.
If you need a brick or set of bricks replaced, I can cut them to size after sweeping your chimney.