Vortex shield is fairly accurate. I have had no issue catching rockets and sniping pilots with them. Its energy bar drains both based on what kind of projectiles you catch, and the amount of time you hold the shield up. In order for it to start to regen (to avoid infinite vortex tennis, and ensure a loser), there is a long delay before it will start regening since last use, which is somewhere around half a second maybe more.
Vortex shield may mess with projectile speed a bit, as when you release projectiles they appear to be in unison. It very well could speed up projectile speed of slow rockets to a uniform speed.