How to extract vocal part from stereo audio signal?
First of all, how the data is encoded in a mp3 file is irrelevant to the question unless you aim at doing compressed-domain processing (which would be quite foolish). So you can assume your algorithm will work with decompressed time-domain data.

The sum / difference is a very, very basic trick for vocal suppression (not extraction). It is based on the assumption that the vocals are mixed at the center of the stereo field, while other instruments are panned laterally. This is rarely true. L-R and R-L will sound the same (the human ear is insensitive to a global phase shift) and will give you a mono mix without the instruments mixed at the center. The problem is, once you have recovered the background, what will you do with it? Try to suppress it from the center (average) signal? This won't work, you will be doing (L + R) / 2 - (L - R), this is not very interesting... You can try any linear combinations of those (averaged and "center removed"), nothing will come out of it!

Regarding filtering approaches: the f0 of the voice rarely exceeds 1000 Hz but its harmonics can go over that. Removing the highest frequency will make consonants (especially sss, chhh) unpleasant. Some male voices go below 100 Hz. You can safely cut whatever is below 50 or 60 Hz (bass, kick), though

Some recent developments in voice separation worth exploring:


Jean Louis Durrieu's background NMF + harmonic comb > filter model. Python code here.
Rafii's background extraction approach. Straightforward to code and works well on computer-produced music with very repetitive patterns like Electro, Hip-hop...
Hsu's approached based on f0 detection, tracking and masking. "A Tandem Algorithm for Singing Pitch Extraction and Voice Separation from Music Accompaniment" (can't find accessible PDF).