Vauthry turned people into eaters at his discretion. Something else we don't see sin eaters doing--if they catch you, you're either dead or turned. Titania didn't have a choice; turning people into sin eaters was a byproduct of them existing. Vauthry can not only choose to not turn people into sin eaters, he can maliciously direct them to attack in ways they won't otherwise do.

So tell me, which arm did Emet-Selch twist to force Vauthry to attack Lakeland again? How did he manipulate Vauthry into flying over in the aftermath, gloating?