What effect does the vertical acceleration have on the horizontal velocity of the projectile?

None, if air resistance is ignored.

Explanation:

At any instant, the projectile has vertical and horizontal components of velocity.

Vertical acceleration due to gravity affects the vertical velocity by accelerating the object toward the center of the earth, and by decreasing the upward vertical velocity ...

The horizontal component of velocity makes the object travel horizontally as long as the projectile is airborne.

Thsi discussion assumes that air resistance is ignored.