I doubt that theory too.
1. Who knows that the upper left part of the first picture shows a vasto? Could be anything.
2. If the form of the mask had anything to say about how close the espada is to becomming a vasto the other espadas would have masks that resembled a vasto as well (we only saw the outlnes of sth that could be a vasto anyway. Nobody actually knows what they look like) The skulls looked more like a part of what they change into when they release their swords.
3. There is no hint that any of the espadas we know will turn into a vasto. I can't see any reason for that, especially after this part of the wikiquote:" Vasto lorde are the least common form of menos, but the exact population is unknown. They are believed to reside within the very depths of Hueco Mundo."
---> There already are Vasto Lordes in Hueco Mundo.