The Vorta are already a genetically engineered species. There's no reason to think that individual Vorta can't be upgraded with special features for specific missions. Maybe Eris's specialty was contact and diplomacy with telepathic/telekinetic races. Maybe the telekinetic ability was given to her specifically for use in the Federation.
And I don't think every Vorta has met or seen a Founder; perhaps Eris didn't recognize Odo for what he is. Of course, being sent undercover would mean she'd have to keep things to herself, so maybe she was just a very good actress.