Got ninja'd by CQ.
But yes, Hamlin was a more staunch abolitionist. His actions may provoke the border states, and Hooket IOTL wasn't all that great. There's a Time Magazine article about the worst U.S. Vice Presidents, and Hamlin was one of them due to his cronyism via patronage and reportedly living a lavish life on the government's time. If that is taken at face value, the Union may suffer more losses due to incompetent leadership.
Should the Union win the ACW with Hamlin still POTUS, Hamlin was a Radical Republican. Expect OTL Reconstruction to be applied, minus the whole impeachment due to presidental support. The 40 acres and a mule might get applied, the ironclad oath will be strictly applied, and ITTL's Reconstruction will be more repressive to former Confeferste whites as long as Hamlin's still President.
From what I understand, Hamlin didn't want to press the 'hang all traitors' issue so he might just have them swear the ironclad oath or have them imprisoned for life. Not sure if he was a staunch anti-death penalty guy, one of the fourm members mentioned it.