Neven didn't want to tell the plan to Vera. No matter how much she pouted or asked nicely, he kept it for himself until the time to depart.
He had understood why Vera would spill the details of the missions one bit at a time: it would make her have some control over the case till the very end, together with people's attention. She would lose that if she told everything at once.
It also was useful for him, since his wife would nag and ask, and ask again. Her attention didn't move away from him for the whole rest of their staying in Paris. Having secrets was indeed rewarding. He would have kept that in mind for the future.
But then, at some point, he had to tell her where they were going and why. Not just because Vera was getting close with her guesses, but also because it was not that great of a secret. She wouldn't be surprised to hear his plan, so she might have become colder the next time he did that.