Type Vietnamese with Telex, then pick the Nôm character that fits.
Type a syllable with Telex, for example viet → việt.
A list of Nôm characters appears below the text field: tap one, or press 1–0.
With ↑↓ you move through the list, Esc closes it.
With QATT on, a syllable you do not pick a Nôm character for stays in Quốc Ngữ until you press Space: then it turns into Quốc Âm Tân Tự. Enter picks the first Nôm character, or converts when the list is empty.