Any firm is in effect a vertically integrated organization, from production to marketing.
If cost considerations justify, the same firm will be broken into different units, rather than forming one firm. Likewise, if two firms operating in different markets but nonetheless related in a vertical value chain, e.g. cable programming producer and cable system operator, they might find it cost effective to merge into one firm. Horizontal mergers are scrutinized because it effectively eliminates competition. It has no framework nor will to deal with vertical markets. But, vertical mergers are as detrimental as horizontal mergers to competition, if promoting competition is indeed still a goal.