The foundation of Vega's theory is that of longing, as he himself states, " Devil fruits are a manifestation of desire to see how different paths of evolution can play out", he goes on to state what said person would think, " Gee I wish I could", " If only we could do", now this is poignant because it explains the backbone of his entire theory. Desire, longing and wishing, the strongest emotion is that of desperation, the deep desire to want something that you can never have, it drives people mad. Vega theorizes that such strong feelings manifest themselves into reality.
An example would be of a boy with the dream to fly, he obsesses over it day after day, until he reaches old age and dies with this desire in his heart. Vega believes that this strong longing would then manifest itself into a devil fruit that when consumed, would give one wings and then they'd be able to fulfill this dream. In Zoro and Sanji's case, they didn't express any desire to manifest these powers, they are simply a physical expression of their fighting spirit.
Even if we say it's born from desire then we are still missing the core aspect which is desperation; they have already made these "desires" into reality, going back to the example of the boy, if we related him to Zoro and Sanji it would be as if he sprouted wings one day after jumping off a cliff attempting to fly, in this case there would be no more longing for it since it's been achieved. The more apt comparison would be of Sanji's wish to turn invisible, this is the closer comparison but we know such a devil fruit already exists.
If I misunderstood anything please clarify it for me.