How do I carry out an ancestry/admixture test on a single VCF file?
A modified implementation of Vivek's answer.

peddy is a Python package that samples an input .vcf at ~25000 sites and projects onto a principal component space built on 2504 thousand genome samples. The author has extensive documentation of the tool's features and a link to the preprint. 

I downloaded the .vcf and .vcf.tbi for the NA12878 sample from Genome in a Bottle's ftp here. Then, created a custom .ped file, NA12878.ped, with the contents:

NA12878 HG001   0       0       2       0

At the command line:

$ peddy --plot --prefix myvcf HG001_GRCh37_GIAB_highconf_CG-IllFB-IllGATKHC-Ion-10X-SOLID_CHROM1-X_v.3.3.2_highconf_PGandRTGphasetransfer.vcf.gz NA12878.ped


The output files all have the prefix myvcf., here's myvcf.pca_check.png