Vector normalized in Godot

vector_example.gd

1º Add the direction vectors. Ie: you want to add the right key + up key velocity.
var vel = vel_left + vel_right

2º Normalize the velocity, this will make the length of the vector exactly 1 unit.
vel = vel.normalized()

3º Now the movement would be only 1 unit per frame, so multiply for the real speed
vel = vel * speed
translate(vel * delta)

4º That will make the movement of the body constant (no drag). But you can normalize the vector only when the speed it's too high:
var vel = vel_left + vel_right
if vel.length() > MAX_VEL:
  vel = vel.normalized() * MAX_VEL
translate(vel * delta)