I'm not sure that's a really good...example? Why would placing a veil on a carrot just to see the carrot even happen? Just peel them, boil them, and throw some brown sugar on the whole thing...sheesh. 
Also, if I am reading correctly, your example is saying that someone would overcome the block aspect of The Sight by throwing a veil on something - in that case wouldn't they be casting the veil through the block? The two actions don't seem to be working together, in that case the block would apply. This is ultimately up to the GM of course - I would hope a table could agree on what constitutes cooperative actions with the Sight though.
However, if someone is attacked by an invisible force I would find it acceptable to open The Sight, identify said force, and then have the ability to counter it while the Sight is open without the block applying. Those two actions seem to cooperate to me and, per the rules, is allowable.
Do you have an issue with the cooperation rules? Do you believe the block should always apply?
You seemed to be arguing in your example that, because the vampire in question was veiled, and thus the sight allowed for easier offensive spellcasting against them, that such casting would not be affected by the block thanks to the 'cooperative' clause.
In my carrot example, I attempted to show the absurdity of that, particularly by way of the relative strengths of the blocks involved.
You say you would just tell the player to close the Sight and peel the carrot, but that just sidesteps the issue, rather than addressing it. The two scenarios are essentially equivalent in all but seriousness. In one you are peeling a carrot, and in the other, fighting for you life, but the issues involved are not meaningfully distinct for the purposes of this discussion.
If your response to one is rendered absurd by the other, than it is incredibly likely that it is absurd in both.
