"th" represents the standard Latin transliteration of the Greek letter θ present in the form Ἐσθήρ appearing in Septuagint for Esther.
Modern English form is (indirectly) coming from the Greek form and not from the Vulgate form Hester.
In Septuagint, all tav yielded θ, cf. Θαμαρ for another biblical female name Tamar ( תָּמָר).