Why can Yasuo block Vel'koz's W but not Xerath's Q?

I think it's because Xerath's Q is an instantaneous skill, it has no travel time - a person who is closer to Xerath does not take damage earlier than a person that is further away. Everyone in the AOE takes damage at the same time. It's kinda like Lux's ultimate; it looks like a projectile, but everyone takes damage at the same time.

Void Rift is different because it does have a travel time, so it is considered a projectile. Although it does seem silly that a rift is blocked by a wall. Visually, things can go over the wall (I think Ziggs ult can fly over it, not 100% sure) so I don't see why things can't go "under" the wall.

I have heard Vel'Koz R goes through the wall.

good its about time something goes through that stupid wall.

and xerath's Q doesn't extend after it's casted (it doesn't send a charge from xerath to the end point, it just makes the charge) , velkoz W opens up and moves forward to an end point

Vel's W has a travel time, while Xerath's Q is a laser with no travel time.

Vel's ult work's similarly to Xerath's Q, and it won't be blocked by windwall either.

That wall....wind would NOT block a bomb from hitting Yasuo, and my bb Ziggs is crying cuz he can't farm :(

A lot of things go through the Wind Wall, for instance Lux's R, Vel R, Azirs Q, Xeraths Q, etc. It doesn't block everything, and without his wall, he wouldn't be able to fight a range champion in lane.

velkoz's w starts as a skillshot, then an aoe, whereas xeraths q is a set line skill, xerath's e can be however.

xerth q is a beam vel koz w is a projectile that leaves a trail behind it

Simple one is a projectile, the other is a laser. The easy way to tell the two apart is to ask "does this ability have a travel time?" Xerath's hits everything at once. while vel'koz's opens starting at him and moving forward (Keep in mind that wind wall will not cancel the second tick if used after the void is already on the ground).

That said I think that the level of power in Yasuo's wind wall is too high. that type of ability should never have been added to the game. (It can negate Ultimates for Pete's sake!)