I am reading the screenplay for Secretos del corazon and I have found a few places where this expression is written "voy a por las vacas" or "baja a por otra"...I can't seem to see the reason for the "a" in these phrases. Is this a new "modismo"? I would never think to use the a in these phrases. Gracias por clarificarla.

It is not a new modismo, it's the standard use. The structure "ir a + verbo en infinitivo" to express intention is pretty standard in both spoken and written lenguage. Another examples:

Voy a por los niños
voy a por las notas
ha ido a por los libros....

Nativo,
Of course ir a +infinitvo is not a problem...it is the two prepositions together
(a por)...

It's standard usage as far as I know - if you miss out the "a" the meaning can change- "me voy por las vacas" would mean "I'm going because of the cows". There's also a standard expression "a por ello", meaning roughly "let's go for it".