Yasuo's crit reduction affects total damage dealt. His crits deal 180% damage of a normal attack, and 225% with Infinity Edge.
Similarly, getting hit by a crit with Randuin's Omen will reduce the damage you take to 180%, or 225% with Infinity Edge.
Of course, Randuin's Omen stacks with itself. So if you have 5 Randuin's Omens and you get hit by a Yasuo attack, you take...
107% damage (200-10% = 180 - 10% = 162 - 10% = 146 - 10% = 131 - 10% = 118 - 10% = 107%)
So Yasuo without Infinity Edge critically striking a guy with 5 Randuin's is practically unnoticeable.