In the main diagram AB is perpendicular to v1, and AC is perpendicular to v2.
If you rotate the line AB anticlockwise by 90 degrees it will be in the direction of v1.
If you rotate the line AC anticlockwise by 90 degrees it will be in the direction of v2.
Make a V shape with your fingers, now rotate your hand 90 degrees.
Both fingers rotate by the same angle so the angle between your fingers doesn't change.
So the angle CAB in the magenta triangle is the same size as the angle RPQ in the green triangle.
Both triangles are isosceles, therefore the other two angles are the same for both triangles.
Of course the length of the sides may not be the same so the triangles may not be congruent.
But the angles are the same so the triangles are similar.