Vertical merger is integrating with a company in the same supply chain. On the other hand, horizontal merger is essentially a company acquiring a competitor.
Vertical is most likely to fail in most cases acquirer is moving out of their business therefore may not lead to economies of scale and in some instances could also lead to diseconomies of scale (for entire value chain to operate efficiently in sync).
Horizontal merger = one business acquiring another business e.g. a competitor
Vertical merger = one business acquiring another in the same supply chain.
Both have equal chances of failure if not planned or executed properly.
In my experience, vertical mergers are easier to integrate because the companies have often already worked with each other as one is a supplier for the other. They already have some familiarity with how each other operate.