I agree with you that we should not presume that Vigano has traditionally-valid Orders. As the Moral Theology manuals say, we must have "moral certainty" with regard to the Sacrament of Holy Orders. I will not support him financially until both he and his consecrators confirm publicly.
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I strongly agree with every word of this.
That said, I think the reason that Vigano doesn't want to make it public could be the "scandal" it would cause to non-Trads, including New Rite priests that might want to join him at his seminary. He may think it is too much for those people to digest at the moment. I am not defending that take on things. But I can see that as a valid concern if he is trying to attract people brainwashed by the Novus Ordo, which I think is his main target audience.
I just don't see it. Why would those people care that much if he got consecrated again, if they didn't think it was necessary? Worst case scenario, they would say he received a sacrament that was unnecessary or invalid, and that it was sacrilegious (though I doubt most conservative Novus Ordos have any concept of such things). But as far as discouraging new-rite priests from joining ... why would it? It certainly wouldn't take away his power to ordain priests, even if it was unnecessary. Whatever sin might be involved in an invalid conditional consecration would be only on his conscience in the worst case scenario. At most they might think it was a bit scrupulous. None of this amounts to an adequate reason for keeping the conditional consecration secret, if it did happen.