True, but those Vehicons he possessed where already pre-built by KSI. And they did indeed need the Seed (Rhymes not intentional) to make more Transformium, as it was the entire point of KSI wanting the seed. The dead Transformers on Earth wouldn't really get him much of an army compared to the 50 he had in AoE. Again, Galvatron can't really do anything without his army aside from throw a tantrum in a city only to get driven off by the Military or the Autobots. Unless he finds a MacGuffin to make Prime come back.