Does anyone know the meaning of this character ()?
The Nom Foundation has a lookup tool that might give a clue:



Although I don't understand which entry belongs to which character, or what the relationship among the characters is (Han Nom was less standardized than Chinese Hanzi, so some of the 4 shown here might even be distinct variants), my best guess based on the expressions is 'mold/mould' as in molding/moulding (or the derived meanings like 'shape/pattern'). The 木 radical might also support the meaning, since molds tended be made from wood in earlier times (additionally, the Mandarin Chinese equivalent for mold/mould is also based on the 木 radical: 模 [mó/mú]).

EDIT: If the two-character expression used in your description is transliterated as 'khuôn mặt' (literally "face shape"), I guess the character in the title means 'shape'.