I'm the NewSoftSerial guy, and I do just what you're describing all the time--read GPS data through a "soft" serial port so that I can keep the "real" one available for debugging and other tasks.
I think that the _vector* symbols probably refer to the interrupt vectors that NewSoftSerial harnesses to perform RX. My guess is that somewhere else in your code someone is trying to use pin change interrupts. Is that possibly true?…"
If you'd like to see an example of exactly such a project -- one that uses NewSoftSerial to gather information from a serial GPS module -- you might take a look at something I built called the "Reverse Geocache Puzzle".