Another solution with tsp and background processes.

# this sets number of max jobs. Here we use the number of processes. You might want to change this to another number.
tsp -S $(nproc)

# rest is similar. we just add tsp to start of the command and & at the end.
# & at the end calls all the processes at once but tsp queues them and calls them in batches.
for file in /vcffiles/*.vcf.gz; do
    tsp bcftools view -Oz -S sample_list.txt $file > /output/subset_"${i##*/}" &
done

let's do it using nextflow, I won't test it so there will be some small bugs, but you get the idea.