Try Heng's https://github.com/samtools/htslib
If you can work in the SAM or BAM formats you can do a lot with the samtools header files.
http://samtools.sourceforge.net/samtools/masterTOC.shtml
On the same topic Bamtools also offers an API
https://github.com/pezmaster31/bamtools
Do you need to process the BCF?
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Unfortunately, I can only be assured of having VCF/BCF files, and not necessarily the original aligned BAMs. I am working on a variant annotator, and I would like for it to able to take either VCF or BCF as input, and produce the same for output.