How can I merge VCFs files with non-overlapping sets of variants but partially overlapping sets of samples? For example because for a certain chromosome I only had coverage for some but not all of my samples.

I can find tools to merge VCFs with identical sets of variants but non-overlapping samples, and to merge VCFs with identical sets of samples but non-overlapping variants. But neither is really what I need.

For example, suppose I have:

1.vcf: variants 1, 2 and 3 in samples A, B, C and D

2.vcf: variants 4 and 5 in samples A and B.