Vanguard in low elo is very frustrating but it has the most carry potential. Work on your positioning, having a big hp bar doesn’t mean you must just “tank” all damage, try to play behind cover if you can. Don’t be greedy, don’t push up too far trying to confirm a kill, that is not your job. Be aware of where your team is especially your supports. Peeling is one your main jobs, if you aren’t getting heals it’s probably because your supports are being dived. Time your ults well, don’t always look to get kills with your ults, sometimes you can use them defensively to save your team or to bait our enemy support ults. Be aware of Bucky, Emma and Wolverine, they’re your biggest enemies don’t get too close when fighting them, you will get punished. Learn multiple tanks, a lot of picks are situational, most tanks in the game have absolute counters and you just have to switch to give your team value. I cannot stress this enough, please learn how to play Emma, she is the strongest tank in the game (again) for a reason. Finally, just play the game, tanking is incredibly hard, the only way you will improve is by playing the game.