The impact of a competitive inhibitor on Michaelis-Mentin activity plots is determined by the substrate concentration. A substrate concentration that is vastly higher than the inhibitor’s concentration would lead to no / a minimal impact on Vmax. Where the inhibitor concentration is elevated, the impact on Vmax is increased, reducing the enzyme’s activity and having a stronger inhibitory effect.
Km increases as more inhibitor is added.
For a given [inhibitor], increasing [substrate] will always increase the enzyme activity due to the competitive nature of these inhibitors.