what's patient 13054's average value of vancomycin since 12/2105?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 13054 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'vancomycin' ) and strftime('%y-%m',labevents.charttime) >= '2105-12'