Don't mind having a go at answering the appropriateness of this vector-based style, from a design/psychological perspective;

It might be inherent to the mathemetical precision of this drawing style. The clean-cut shapes and geometrical shapes imply a strict adherence to rules, purposeful determination and look clean, clear and predicatable.

In short: they make your product look well thought out.

As opposed to a sketchy drawing style. (Sketchy. What's in a name?)