extracting last few lines of a growing bgzip compressed .vcf.gz file
27GB is not huge. You can zcat the file and see the last few lines. It will take ~10 minutes. It is theoretically possible to peek a bgzf file, but no such tools are available.

By the way,


  I am running bcftools merge to create a single .vcf.gz file out of a collection of chromosomal .vcf.gz files from the 1000g project.


Probably you should use bcftools concat, not bcftools merge. merge combines VCFs from different samples. concat concatenates VCFs for individual chromosomes; each input VCF contains the same set of samples.