if they set/adjust the vector manually every frame
Well but the vector is set to a fixed value so deltatime shouldn't matter, right? Deltatime is for acceleration.
e.g.: Set vectorX to vectorX+acceleration*dt
But OP does: Set vectorX to value*maxSpeed
The only factor that changes really might be the movement angle, but assuming the player just holds right on the stick, the result should always be the same value regardless of framerate. e.g. (cos(0) = 1) * (maxSpeed = 300) = 300
And any change in angle would just register instantly on that tick, not as an acceleration.