Let's start by defining our terms. A vector sum is a sum of vectors. A vector (in introductory physics) is a quantity with magnitude and direction; as opposed to a scalar with only magnitude. A net force is the sum of forces acting on an object/system. For example, if the net force on an object is zero than the object also experiences no acceleration.
So, to answer your question. A net force is a sum of forces and forces are vector quantities. Therefore, a net force is a sum of vectors or a vector sum.
Thank you for your question and I hope this helps!
-MJM
