Rotating vector relative to direction

This is probably absurdly simple, but I'm trying to find specific coordinates in a 90 degree (2d) in front of a relative vector. I know I can get relative forward with (destination - origin).normalized * distance, but given that I don't have a transform to explicitly rotate, how do I calculate a Vector3 which rotates that point n degrees around the destination vector?

 

Code (CSharp):

1. Quaternion.Euler(0, 0, 90) * (destination - origin)

 

Ooh derp, for some reason I had discounted quaternions entirely and was messing around with the dot product. Thank you so much!!