I'm trying to remember the last big acquisition that actually helped a team win the cup, Vemette and Chicago a few years back?
Looking at the last 5 years:
2017 year Pens added Streit and Hainsey. While Streit was irrelevant Hainsey was arguably their best dmen and logged a ton of minutes.
2016 the Pens added nobody at the deadline but did trade for Hagelin in January.
2015 as you said the Hawks added Vermette
2014 the Kings added Gaborik
2013 the Hawks added Handzus
Some are rather insignificant adds but there are definitely times where without the addition the team that ended up winning probably wouldn’t have won. LA doesn’t win without Gaborik in 14, Pens don’t win without Hainsey last year and while Vermette was mostly useless for the Hawks he did score the backbreaking Ot goal against us.