All things considered I'd give this to Vader, but only if psychic attacks prove to be useful against Steppenwolf, and if a lightsaber can cause damage.
Otherwise, Steppenwolf wins an all out physical battle here. Once Vader was put into the suit, he had to adjust his entire fighting strategy to focus his main advantage to be brute strength. He wasn't agile or athletic anymore so he often just beat his opponents into submission with hard, relentless strikes. But Vader can't compete with Steppenwolf using strength here. One blow from Steppenwolf here and Vaders done, and I think he can only doge and evade for so long. Telekinetic tricks, throwing objects and such, will be a temporary distraction.
Plus keep in mind Vader can't use force lightning, which would otherwise help him out tremendously here. That said, if he cause use feats such as Force Storm and Nightmare. I only say if Because I can't say for sure if Steppenwolf has any inherent immunity to psychic moves, though as a battle hardened soldier and general or whatever his title is, he probably has training in some form of resistance.