It should also be pointed out that Moses did not put a vail over his face because HE had some kind of problem. Rather, he put the vail on when he spoke to the people of Israel. But when he went to speak with the Lord, he took the vail off:
And it came to pass, when Moses came down from mount Sinai with the two tables of testimony in Moses' hand, when he came down from the mount, that Moses wist not that the skin of his face shone while he talked with him. And when Aaron and all the children of Israel saw Moses, behold, the skin of his face shone; and they were afraid to come nigh him. And Moses called unto them; and Aaron and all the rulers of the congregation returned unto him: and Moses talked with them. And afterward all the children of Israel came nigh: and he gave them in commandment all that the LORD had spoken with him in mount Sinai. And till Moses had done speaking with them, he put a vail on his face. But when Moses went in before the LORD to speak with him, he took the vail off, until he came out. And he came out, and spake unto the children of Israel that which he was commanded. And the children of Israel saw the face of Moses, that the skin of Moses' face shone: and Moses put the vail upon his face again, until he went in to speak with him.
(Exo 34:29-35)
The vail was a barrier not between Moses and God, but between Moses and the people. So Paul's argument has to do with the PEOPLE being blinded when they "hear" from Moses. Not because of any fault with Moses, but because of themselves.
When Paul says "nevertheless, when it (the heart) shall turn to the Lord, the vail shall be taken away", he is referencing when Moses went in to speak to the Lord "face to face" without any vail. So that Paul is saying the unbelieving Jews are like the Israelites in the wilderness - they only deal with God through Moses behind a vail, hiding the display of God's goodness. But if they will turn to the Lord, as Moses did, and enter the holy place, the vail will be removed and they will encounter God just as Moses did, with no intermeddling vail hiding the glory of God.
As expressed also by Paul here:
For by one offering he hath perfected for ever them that are sanctified. Whereof the Holy Ghost also is a witness to us: for after that he had said before, This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them; And their sins and iniquities will I remember no more. Now where remission of these is, there is no more offering for sin. Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus, By a new and living way, which he hath consecrated for us, through the veil, that is to say, his flesh; And having an high priest over the house of God; Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water. Let us hold fast the profession of our faith without wavering; (for he is faithful that promised;)
(Heb 10:14-23)
So, again, the problem wasn't with Moses, but with the people. Just as Paul explains the problem with the old covenant wasn't with the old covenant in and of itself, but with the PEOPLE:
For what the law could not do, in that
it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh:
(
Rom 8:3)
The weakness of the law was not in it's own self, but in the flesh of the people exposed to it. The law could not produce righteousness because the PEOPLE were hardened (blinded) in their hearts. The new covenant however overcomes that problem of our flesh via the cross and Spirit regeneration:
That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
(
Rom 8:4)