As the title says, you are able to compose the ViewMatrix and the ProjectionMatrix into one instead of multiplying a vector by the individual matrices.
Instead of doing this at run time for each item: Vector * ViewMatrix -> Vector ProjectionMatrix
Do this at the start: ViewMatrix * ProjectionMatrix -> ComposedMatrix
Then when drawing vectors: Vector * ComposedMatrix
This would minimize the number of necessary matrix multiplications. Is there any reason not to do this?