Vista is stronger than Ace. Despite being 2nd commander, he was actually a newbie commander when Vista is a veteran commander being part of WB's crew since Roger's era, strongest swordsman in the WB crew, someone Marco was fully confident in his prowess to the point he was calling him for important tasks(stalling Mihawk, assisting him against Akainu and so on), fast enough to intercept Mihawk mid-attack despite being far away from him, agile enough to jump kilometers above marineford to cut cannon-balls, fast enough to move at Marco's flying speed to intercept Akainu, possessing haki strong enough to actually affect Akainu despite being covered by magma(something that burnt Ace despite being made of fire). Fire isn't a problem. Kinemon can cut fire to some extent. Clearly for the strongest swordsman in the WB's crew, he should be able to do much more than that
Clearly unless you're blind, Vista was portrayed to be one of the top commanders(with Marco and Jozu). Ace on the other side was portrayed to be weaker than yami Teach who was not on the level of top commanders. Even before he actually faced Teach, clearly they tried to prevent him at all cost to fight him because they knew he would lose(and likely they weren't even taking the yami yami fruit into consideration considering they had no way to know how it affected his combat abilities). The truth is Ace became a commander of the second division only because the post was vacant. Though i'm not denying Ace's strength. I like him too much to try to deny his strength, but let's be real here. He wasn't portrayed to be Vista/Jozu let alone Marco level. Though i'm pretty certain that had he survived, at this point, he would have been admiral level or maybe even above. He had such a great potential