Question

explain the relationship between contours of constant potential and the electric field direction.

why is the electric field a vector quantity while the electric potential is a scalar?

Answer 1

E is a gradient, change of potential vs distance. Gradients are perpendicular to equipotential lines.

E is Force/q, and force is a vector (directed) quantity.
Potential is not directed, but in an energy measure in relation to a charge at a distance. Energy is not a vector quantity.