What does soy una vieja mean?

"soy una vieja" has different meanings in different countries

soy una: I'm a

and "vieja" in a strict way means "old woman", but in different slangs means "women", "wife", "mother", or its used in between weman to call another women.

so depending on the context it could mean, "i'm a woman or girl" (putting enfasis in the fact that she is a female), or "I'm a old woman" (putting enfasis on her age)

hope it help