In general we say inertia is represented by mass. Mass is a scalar. It does not have a specific direction and does not follow law of addition of vectors.
Force is a vector quantity. It has a magnitude, it has a direction as that of the change in linear momentum. Force follows the vector addition law (parallelogram law of addition).
Linear Momentum is a vector quantity. It has a magnitude, it has a direction as that of the linear velocity. It follows the parallelogram law of vector addition.
Often inertia means the resistance to the change in state of an object. Mass is the quantity that resists a body from changing its state from rest to motion. That is why it is often called the inertial mass. Mass is a scalar quantity. It does not have a unique direction and it does not follow the laws of vector addition.
Also, when a body is in motion, inertia of motion is represented by its momentum rather than its mass. In that case the inertial is a vector, as momentum has a direction and follows the vector addition law.