I'm on campus today reading through John again in the Vulgate. I love studying the different "styles" of the New Testament authors, don't you? I define style as the tendency of a writer or speaker to consistently choose certain structures over others available in the language. Of course, such choice is not always conscious. Style can be intuitive or conscious. In John 21:15-17, the Vulgate obviously felt it was important to translate the Greek verbs agapao and phileo differently, even though John's style would almost require us to see these two verbs as practically synonymous. Note below how both amare and diligere are used.
This issue has been discussed for centuries. Some acknowledge a duality here, saying that, for John, a variety of surface forms is possible. Hence the two verbs are synonymous even though they have different forms. The opposing position is that every difference in form brings with it a difference in meaning, and that true synonymy is therefore impossible.
I don't propose to resolve this issue here. I would simply say this: The more you understand the linguistic system of Koine Greek, the more you appreciate the infinity and variety of possible choices and even combinations of choices available (not only are there two Greek words for "love" in this passage but two words for "know," two words for "sheep," and two words for "feed").
Is there a specialized vocabulary for John? There may very well be. Does John enjoy using synonymous words synonymously? Apparently so. The question is how consistently he does this. Only a good feel for John's style can help us here -- hence the need to read as much of John in Greek as possible.