Va de vencida

Va de vencida
 

Hi,

Comenzaba este largo siglo, que ya va de vencida.

Why not vencido?

This one I found on the Web: "Vencida" es una forma de "vencido", un adjetivo que se puede traducir como "defeated".

Expired, too, I suppose.

I can't really understand this o/a thing :)

Yes, vencida is the feminine equivalent of the masculine adjective vencido.

However, google translate indicates that de vencida is a set expression that usually means "overdue" and de vencido is a set expression that usually means "expired".

In your original sentence, it looks like the prepositional phrase de vencida is an adverb modifying the verb va.

A possible translation of your original sentence is "This long century began, which is already overdue."