Is it impossible for Goku and Vegeta to turn Legendary Super Saiyan?
Vegeta's har does turn green. Also, when Goku transforms into Super Saiyan Blue, there is a tinge of green aura. I believe this has to do with the animation and it isn't a new transformation.Broly's Legendary Super Saiyan transformation is similar to that of Kale's. If you look at the way these two attained their respective transformations, they both originated similarly to the regular Super Saiyan transformations. In the sense, Broly transformed to this level when he was angry and struggling against Gogeta and Kale transformed to this level when she was angry.Hence, I believe Goku and Vegeta cannot use this form and the transformation is solely based on the genetics of the Saiyan.