VEP: correctly use configured plugin fields
VEP: correctly use configured plugin fields

Fixes #1705

biocyberman/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,brainstorm/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,biocyberman/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,brainstorm/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen