Database: academic
Question: return me the number of papers in VLDB conference in " University of Michigan " .
SQL:
SELECT COUNT ( DISTINCT t5.title ) FROM organization AS t3 JOIN author AS t1 ON t3.oid  =  t1.oid JOIN writes AS t4 ON t4.aid  =  t1.aid JOIN publication AS t5 ON t4.pid  =  t5.pid JOIN conference AS t2 ON t5.cid  =  t2.cid WHERE t2.name  =  "VLDB" AND t3.name  =  "University of Michigan";