This study contributes to a longstanding discussion on the status of aspectual morphology and the aspectual architecture in Slavic by investigating aspectual properties of perfective and imperfective VP idioms in Polish. The investigation reveals that only lexical prefixes which can result in idiosyncratic meanings are part of basic perfective VP idioms. Building on the idea that little v is a demarcation line for idiomatic meanings, it is concluded that lexical prefixes are vP-internal. It is also shown that basic imperfective VP idioms are compatible with some superlexical prefixes and basic perfective VP idioms are compatible with secondary imperfective morphology, which suggests that such morphology is vP-external. Further semantic analysis of the interaction of basic imperfective VP idioms with different classes of superlexical prefixes shows that the compatible ones measure over some scale associated with vP-external material or the temporal trace of an (idiomatic) event. In contrast, the incompatible ones measure over a scale encoded by a verbal predicate or impose semantic restrictions on its argument and hence they intervene in the semantics of a VP idiom. Based on stacking facts and the interaction of different classes of superlexical prefixes with secondary imperfective morphology, two classes of superlexical prefixes are distinguished: high (projected above secondary imperfective) and low (projected below it). These observations are compatible with the view that aspectual morphology is separated from PFV and IPFV operators. The former may merge lower in the hierarchy, and the latter act at the level of AspP as phonologically null operators.