Has he tried to fool a plant, though, any plant? Because that is different than facing humans and letting them believe something that they would rather believe. Even Vash knows that.

I was not expecting you to do it! But you know more about our powers than I do.

She says that the sisters here will help.

The worst part is that Vash is also bracing himself for it. But unless Tessla herself wants it, he will fight against anyone who tries.