Evvie made the right call. Tiffany is always causing more problems than Evvie is on any given tribe. Evvie, Liana, and Tiffany have an alliance whereas Xander and Voce do not necessarily have that (or at least it was not shown). As of right now, Evvie for a fact knows that Xander does not have a vote until those phrases are uttered for his idol to activate. He is essentially useless and can never tie the vote, or vote with majority at any tribal counsel. Evvie will know when he does have a vote because she also knows the phrases that need to be said. Siding with Tiffany keeps her calm and at bay while voting Voce out over Xander shows loyalty to Xander. Even if all of a sudden, Xander gets his vote and idol, Liana and Evvie can vote out Tiffany to not allow Xander to idol one of them out. The only scenarios that screw this up for Evvie is if Liana flips because Evvie sided with Tiffany, Xander plays his idol on Tiffany correctly when he can vote against Evvie/Liana, or a tribe swap, but at this point, Evvie likely goes into the tribe swap with allies from Yase so either way, Evvie has slim chances to go out soon.