Database
[{mainVendor:"mainVendorObjectId",vendors:["secondVendorObjectid","thirdVendorObjectId"]},{mainVendor:"mainVendorObjectId",vendors:[]},{mainVendor:"mainVendorObjectId",vendors:["secondVendorObjectid","thirdVendorObjectId","forthVendorObjectId"]}]
Query
db.collection.update({mainVendor:"mainVendorObjectId"},[{$set:{mainVendor:{$cond:{if:{$eq:["$vendors",[]]},then:null,else:{$first:"$vendors"}}},vendors:{$cond:{if:{$eq:["$vendors",[]]},then:[],else:{$slice:["$vendors",1,{$size:"$vendors"}]}}}}}],{multi:true})