Anakin Skywalker would be a light side hero. He's generally considered an agile and powerful but somewhat reckless light saber focused Jedi. You'd expect him to have dash attacks and a powerful melee strike, possibly one that uses some of his own hp to perform, gives him a debuff, or has a long recovery animation. At any rate you'd expect Anakin to have a completely offensively tooled kit and have no defensively focused abilities, making his play-style distinctly different than Luke who is more defensively tuned.
Darth Vader is obviously a darkside villain. He's generally considered a tanky, slow for a force user, but powerful and of course light saber throw and force choke are his signature moves.
Vader has a different body type than Anakin, a different fighting style, and obviously a different take on the force.
Despite there being a brief transitional time where Anakin becomes Vader, the two are typically considered two different characters. If they just gave Vader an Anakin skin rather than making Anakin a completely separate hero it would disappoint a lot of fans. If you had Anakin moving and fighting like Vader it would just look terrible to be honest.
