What do you think?

Once the bomb was dropped and humanity started to increase, would they still recognise the old boundary lines between the USA and Mexico? Is there a possibility that if the NCR grew as the major influential power of what was south west USA that they could push into parts of Northwest Mexico without even seeing it as a different country? Mexican people have always been an influence in the southern states of America so ethnically they are no different to many US survivors.

There's no reason for the old borders to be respected, although it wouldn't surprise me if the Rio Grande remains an important border.

Depends how messed up Mexico is... We have very little information on the U.S. let alone mexico or canada