To me:
Vaquero horses are Iberian horses. They hold themselves in a very different way than say, warmbloods or QHs. There’s no comparison between these three general categories of horse in how they will want to hold themselves and use themselves.
The bit and size of the bit are very different for individual horses. Some horses go well in, say, a Dr. Bristol. When a properly fitted Dr. Bristol was introduced to my mare, she hated it because she had a fat tongue. She hung on fat bits. She objected to single joint snaffles. She went best in a Sprenger Training bit (they no longer make) at 14mm (which is considered a thin bit, but for her particular Trak small mouth, it fit her mouth well) and has 2 joints. It is designed to lay across the tongue. Her double bit included a similar snaffle. It was technically too long for her mouth, but it allowed me to really move this simple bit in her mouth and she responded very well.
My current Westfalen gelding also had a relatively small mouth and goes well in this Sprenger 14mm , 3-joint bit. I started him in a fatter bit and he hung on it, Lordie!