At level 14 Yasuo has 30 MR, 1509 health, and a 290 health flow shield. The MR means Veigar does 76.92% of the damage he normally would.
Let's assume Veigar has maxed QW and has 2 points in R. Veigar's base damage on R (the supposed one shot) will be 375 + 100% AP. I will disregard the enemy AP ratio, as you were Yasuo and hopefully didn't build like a moron. Rod of Ages and Luden's add 200 AP (max RoA stacks), with Luden's also adding a 100 +10% AP ratio.
So far, Veigar has 475 +110% AP damage and 200 AP. This is 695 damage. Discounting both Yasuo's flow shield and Yasuo's MR, that means Veigar needs to have farmed 740 AP from Q. If we count his MR, 1152 AP from Q. If we count his flow shield and MR, 2126 AP from Q.
If you let Veigar farm up over 700 minions + monsters (and likely multiple champions) you entirely deserved to die.