I think the 'rules' on this are just guidelines, really. Sometimes the adjectives seen strange together so you should describe it another way.. They depend on the situation. You wouldn't say 'a traditional new train'. For example. You would say za new train in the traditional style' or something.. In this case I would say 'an old traditional Greek vase' because it's a traditional Greek vase that is old.

They have different meanings or nuances. ""a traditional old Greek vase" has the emphasis on traditional - old Greek vases follow a tradition, and this vase falls withing that tradition. in this case "old Greek" suggests an era in Greek history "an old traditional Greek vase" has the emphasis on old (when it was made) - it is a traditional Greek vase, and is not recently made (using the tradition for making Greek vases, I can make one today in that style and it would be old, not new)