To be even more specific, the way vape limits movement speed whilst attacking seems to be mimic just not sprinting, but it as actually faster than vanilla walking, and therefore can be flagged on more precise anticheats (such as karhu). A method to fix this would be making the killaura sprint but slowdown upon attacking (like how normal minecraft functions when you are running at someone and clicking on them)
No that is not why at all. It doesn’t touch your movement speed at all. When you attack your movement speed is slowed, that’s a default Minecraft mechanic. There’s not any deliberate attempt being made to slow you down.
You can make it slow down on purpose for when youd be strafing at your "silent" angle. Thats what other clients do but its possible to detect that, it doesnt account for the fact that you can only move in 8 possible directions relative to your yaw. Each direction has its own speed and your movement will move by an expected amount based on that and your angle. Anything that doesnt completely respect that regular movement can potentially be detected, but its probably not worth any servers effort to check that so far.
So the tiers of this go, full move speed -> mostly slowed for strafing -> only moving in legitimate directions relative to silent angle.
The last way can be steered towards where you're trying to go however its a bit jarring, but its not detectable. You'll be left with this in the end after the other ways get patched everywhere.