A Vespiquen that used its "Mach Wind" attack on my previous turn was hit by Grimer's "Sticky Liquid" attack during my opponent's turn. In this instance, what is Vespiquen's retreat cost during my next turn?

It has no Retreat Cost. When effects that increase something like Retreat Cost by a particular amount are stacked on top of effects that make something like Retreat Cost vanish, the vanishing effect takes priority.

Relevant cards