@Setanta,
Just off the top of my head: Embedded in the definition of 'superior' is the notion of 'better than the other(s)' in one or another respect. Depending on the context, it could include 'best among all'.
When 'superior' is preceeded by the definite article, we get, for example, 'the superior team', which is clearly superlative. Just substitute a more well-known superlative like 'best'. If there is no meaning lost, its equivalent and thus, a superlative.
Without the definite article, we have 'Vietnam's team is superior to', which is comparative just as saying 'VN's team is better than'. Substituting a more well-recognized comparative results in no loss of meaning, thus they are equivalent comparatives.
Seems to me that context is crucial. A lot of people want a black-or-white, absolute answer, and language often doesn't work that way. It's not like a programming language where each term has only one clearly-defined function. (I know jack about programming, btw.)
Anyway, that's the way I'd explain it to my students. I admit that I haven't done any research on it, so I'm open to correction by someone who has.