Document image vanishing point detection and evaluation
why use OCR techniques that are sensitive to vanishing points?  

projective distortion just skews the letters- right?  So, why not just model those expected skews into the OCR templates for each letter?  Also, the nearby letters would skew similarly, so you could probably use correlated parallel lines among neighboring letters to estimate the amount of local/global skew (and avoid dealing with VPs).

also, I assume you mean the phone will take a pic of the document, so don't you also have to worry about the paper not being flat and having curve linear distortions too, which seems to me would completely confound, and supersede any vanishing point issue you are worried about.