Is it permissible to name a subsequent newborn after the father, as fulfillment of a vow the questioner made to himself, after his wife miscarried a male fetus whom he had intended to name with the same name?

There is no harm in naming the newborn with the same name as the firstborn. It should be noted that the Hadith "The best names are those that begin with 'Abd (servant of) and those that are praised" is not authentic. The correct one is: "The most beloved names to Allah are 'Abdullah and 'Abdur-Rahman." It is also recommended to name children with the names of prophets and righteous people.