I have a similar experience, funnily enough it was a Vlad player also. Anywho it was a teambuilder game and I saw that they had a platinum Vlad against our unranked Talon, my immediate thought was GG matchmaking Rito followed by groaning at the thought of this Vlad getting really fed and dominating the game and being an ass in all chat. Boy was I wrong. Vlad started talking to us in all chat throughout the game, fairly early on wishing us "GL HF :)"and the like, meanwhile I had Talon flaming me for not ganking enough and it was "too late now" because Vlad hit level 6 or 7 and i'd only ganked his lane 3 times so far. The Talon started flaming pretty much everyone on the team and it spilled into all chat, Vlad came to our defence telling him to chill out as it was only a normal game afterall.
Anyways we were getting beat badly and the enemy got baron and made their way onto our Nexus, the enemy Thresh decided to take it upon himself to say "gg easy" as they were taking our nexus turrets, unfortunately for him we all got back up and put a stop to the win, "doesn't matter we won anyway :)" (anyone notice how it's always the worst players on the team that trash talk?) anyways this must've lit a fire inside everyone on our team because we spent the next 20 minutes fighting off backdoor attempts on our open nexus while picking up kills and securing objectives and just hanging in there.
By this point Vlad told us Thresh had started flaming his own team now, anyway we had a very long fought game of just trying to survive and it must've been about 20 minutes and 2 barons to our team later that we managed to ace them and have enough time to push down mid and take their nexus for the win, as we were on the way there Vlad said "To be honest i'm glad we lost, you don't even deserve to win Thresh." And i've gotta say even though it was only a teambuilder game it was one of the most satisfying wins i've ever had and props to Vlad for being incredibly chill for the entire game through both the good times and the bad.
Makes me feel a tad guilty that originally I was discriminating him based upon his rank in relation to ours and assumed he'd act like an asshole, nothing was further from the truth <3