As far as I know, the letter "f" has never been used in the Vietnamese script, and the /f/ sound has always been covered with the "ph" letter combination. Trying to find out why led me to this picture of p. 251 of Alexandre de Rhodes' 1651 dictionary, showing the section for the letter "F" to be empty save for this note:
Omnia que conscribi possent per f. conscribentur cum ph. aptius ad pronunciandum hoc idioma.
which seems to mean:
Everything that could have been written as "f", shall be written as "ph", more befitting for pronouncing this language.
This makes no sense to me. I can't think of any context where the use of "f" and "ph" would have been any different, and I can't seem to find any explanation for his reasoning. IS there any known reasoning? Or is this an example of Rhodes trying to be fancy for fancy's sake?
This is very very far afield from my areas of expertise, but in some quick reading about Quốc ngữ, I see that in the time of Alexandre, aka "Middle Vietnamese," the sound represented by his /ph/ was pronounced as an aspirated bilabial stop, like English "pin," and not like the voiceless labiodental fricative (/f/), English "fin." He almost certainly recognized that this same pronunciation/spelling was standard in ancient Greek and Latin, where words like philosophia were pronounced "PI-lo-so-PI-a" and concluded that ph- was a better representation of that sound than f-. I know nothing about modern Vietnamese but suspect that the old pronunciation of /ph/ has shifted to /f/ in the succeeding centuries.