Killscreen handles shit based on mass per unit time, not the speed of the projectile. It handles Sniper Rounds from Railguns.
*"Upgrade - Killscreen Generator
Given its duty to lead the charge against even technically advanced enemies, the Vanguard requires a defensive system to protect against enemy fire. That system is the killscreen generator. This projection spines on this weapon emit unstable streams of dark plasma particles that vaporize incoming enemy projectiles. The killscreen can only vaporize so much mass per unit time, however, enabling enemies to overwhelm the system with large numbers of projectiles."*
As for the limits for what it can actually vaporize, I have tested it defending against bullets, acid, and facehuggers themselves from the actual description. While this does sound like an assumption, I would say that Vanguards would handle Plasma projectiles as well since I have documented Spear Masters tanking Plasma Shots from Hunters, Pred-Guns, and even other Spear Masters with Plasma Glaives.
I want to give you the benefit of the doubt on Bobaâs Sonic resistance, but again thereâs no hard proof.