Solution
bam_for_ASE_ch
    .groupTuple()
    .phase(vcf_for_ASE)
    .map{ left, right ->
      def sampleId = left[0]
      def bam = left[1]
      def bai = left[2]
      def vcf = right[1]
      tuple(sampleId, vcf, bam, bai)
    }
    .set { grouped_vcf_bam_bai_ch }