Uh, yeah, when I'm playing Vlad
Someone who is weak at fighting in lane in exchange for an amazing lategame?
get one shot the second he hits level 2 (from full health)
Sounds like he got level 2 first. Why didn't you respect it? his dash is fairly short range and if he got 2 first it means he is pushing. Just back up.
He was 11/0 until the entire team ganged up on him
You mean 10 more people died before it occurred to anyone to fight as a group? What ELO are you in?
Oh and by the way, Xin can't one shot Vlad at level 2
Xin Q: 45 + (360% AD) if he hits all 3
Xin E: 70 + 60% AP
Xin level 2 base AD: 59.9
Fervor damage per stack at level 2: 1.6 I think?
I'll give him +15 AD from runes and a longsword, as well as natural talent. So at level 2 he has 25.5 bonus AD and 0.7 bonus AP. Total of 85.4 AD
This puts his Q damage at 45 + (307.44).
His Q is an attack reset, and knocks up the opponent, so he gets a free auto before and after his Q. 2 autos is 170.8 damage.
His E does 70.42 damage
Fervor is a bit tricker to calculate for
Charge applies 0 and stacks to 1
A1 applies 1 and stacks to 3
Q1 applies 3 and stacks to 5
Q2 applies 5 and stacks to 7
Q3 applies 7 and stacks to 8
A2 applies 8
Total stacks applied: 24
Damage from stacks: 38.4
So his total damage from this all-in is 561.64 physical and 70.42 magical
Vladimir has 25.4 base armor at level 2 and should be running armor yellows especially vs xin. He should also be starting doran's shield or cloth armor. Let's investigate both. We'll assume he has no AP in his runes or masteries (movespeed quints ftw), so no HP from his passive
Doran's Shield is tricker so let's do that first. Dshield start Vlad has 665 HP and 34.4 armor. The Dshield passive procs on each auto and Q. I don't think it applies on his E but I'm not sure. Assuming it doesn't, the Dshield blocked 40 damage from Xin during the fight.
This puts Xin at 521.64 physical and 70.42 magical, for a total of 592.06 raw damage. This already isn't enough to kill vlad, and we haven't factored in resistances. 34.4 armor means 25.59% mitigation. That reduces his combo to 388 damage. Vlad has 30 base MR all game, and let's assume scaling MR runes because this gives the advantage to XIn. Vlad has 33 armor at level 2, meaning Xin's E does 53 damage.
The Xin all-in against the Dshield Vlad does a total of 441 damage of Vlad's 665 HP. Not a one-shot
Cloth armor is easier to calculate. Vlad has 49.4 armor at level 1 with cloth armor and runes. This puts Xin's post-mitigation damage at 385 physical and the same 53 damage from Xin's E. That's 438 of his 585 HP.
tl;dr your story is bullshit. Either you weren't at full HP, you were a level behind or you failed to itemize and rune properly against your opponent.
tl;dr get your head out of your ass and stop blaming the champions for your own mistakes.