Read the Sanger How-to page to make sure your vcf file meets all the requirements for Sanger imputation.
The preimputation steps for UMichigan and Sanger are relatively similar. The main difference is that VCF files are not split by chromosome under sanger imputation. I recommend carrying out the Michigan preimputation steps anyways. If you have already carried out the preimputation steps for U Mich imputation then the only other requirement will be to merge the chr.vcf files into one vcf file and sort the result.
#merge vcf files from list of files
bcftools concat --file-list ~/preimputation/vcf_list.txt -o merged.vcf
gzip merged.vcf