As you said, I don't believe it would have made the slightest difference for Dani if she had won the veto. She would have to be incredibly stupid to believe that deal. No, Jeff had one thing and one thing only to save him if Dani had won veto and that was Rachel's presence. If Dani had won veto and the subsequent HOH, Jeff and Rachel would have been nominated. Therefore, Jeff needed votes to save himself.
Making a deal with Porsche, especially if she was convinced he'd nominate her, would have secured a vote against Rachel. And what if Dani had won the veto and saved one of her allies? Jeff would have been forced to nominate Adam thus endangering a second vote in a potential showdown against Rachel.
I believe that Jeff would have been voted out if he had faced Rachel in a vote under these circumstances while he would have easily won that vote if he had nominated Danielle first.