To explain the why: VCF files may be generated by two different assays - Genotyping and Sequencing.

While the former costs a company in the range of 50$ in material and personnel costs, the latter is in the range of 200$-500$, depending on the amount of data generated. Business models vary, such that a company may ask a customer to pay the full price upfront or initially offer lower prices to later charge for additional services.

.bam files store sequencing data and when a VCF file is generated from it, a lot of information is lost: Imagine a high resolution photo taken by your camera that you scale down to 100px. You can not go back from there to a detailed 5000px image again. You can maybe impute information, but not bring the original information back.

Therefore: Whatever you do now, even if you manage to create a *.bam file, the genetic correspondences will not improve over the VCF that you already have. In case you ordered a sequencing, the more detailed information is on record, but you will not get access to it without covering the additional costs.