What is the relationship between messaging and the unique value proposition of a feature? Which encompasses the other?

This is a really interesting question.

The way to think about this is somewhat as part of building blocks.

The company's positioning is at the foundation. The value positioning is often core to this. Then often different features are proof points of this value positioning for the product or company.

The messaging for the company, product and feature should build upon this foundation.

The value proposition of a feature is part of a messaging framework. I would go so far as to say that clearly articulating the value prop is potentially the most important part of feature messaging. This is the "why" that end users need to care about to adopt the feature. If users don't understand why a feature is adding value, they're unlikely to use it.