I don't think it's so much that they want to work with Vanessa, but James and Meg know that if Shelli ever wins HOH again, she's coming after the two of them. Vanessa's target most definitely is Becky and Johnny Mac. Just because Becky is HOH doesn't mean that people have to play the game to her advantage. They said they would protect her in the game, but they have to play their own game and if that messes up Becky's game, oh well.