In reply to:

... haven't got anywhere - so am hoping that you, o tsuwm the magnificent, can help:

What is the word used to describe the wedding of two vowels as in Phoebus, encyclopaedia and other words? Or, in
different words, what is the generic term for these conjuncted vowels?

you rang?
the word is ligature, sometimes also called digraph.
here's a link which speaks to the disappearance of same:
http://www.xrefer.com/entry/299261