User blog comment:Zwann/Vasto Lordes in the Espada/@comment-1454424-20091002101844

I do believe that Starrk and Barragan were Vasto Lord class arrancar. The problem really is that they lost and didn't take anyone with them. People simply assumed that Vasto Lords were so powerful that they would one-shot Yama-jii. Hell, people actually believe that Harribel had a chance against Yama-jii when she couldn't finish Hitsugaya.

This "hype" was only in people's minds. As you said, Hitsugaya didn't say they were stronger then the strongest captains. Just that ten of them would be almost impossible to deal with. So the reason people do not believe Starrk and Barragan are Vasto Lord is a combination of underestimating the skills of the Gotei 13 captains and over-hyping the Vasto Lords.

However, what we must always keep in mind is that it is speculation either way, although in this case the evidence is more on the side of them being Vasto Lord then not. I am personally looking forward to some official word from Kubo on this matter. Perhaps he'll mention something in their profiles...