Assuming that the Y axis is up by default (without rotations), then the math is the same all the time. I'm also assuming a KinematicBody2D. When you orient your node, the local Y axis will orient the same way as well. If you rotate your node so it's pointing to the northeast, the locate Y axis will also be pointing northeast.
To impart a bounce to your motion vector:
vecMotion += bouncer.transform.basis.y * nBounceForce.
If you're using a RigidBody2D, then you impart the bounce using one of the impulse functions rather than tracking the motion yourself. But the force calculation is the same either way.