Vergil could interact with the Transformers in a very competitive way. Given his pride and his constant pursuit of power, he might see the Transformers' strength and technology as something to overcome. He could challenge them to duels or races, depending on what abilities the Transformers in the fanfiction possess. On the other hand, if he senses they have a common enemy, he could be more collaborative, sharing his combat skills and learning from their technological prowess.
He might be cautious at first. Vergil is used to dealing with demons and the like, not giant robots. But once he realizes they're not a threat (or if he decides they are an interesting challenge), he could start to communicate. Maybe through some form of telepathy or just by gestures.