your vlookup is the error
this formula 
Range("P3").FormulaR1C1 = "= VLOOKUP(C[-3],MISSINGUSERNAMEDEPT.xlsx!R1:R1048576,2,0)"
generates this 
= VLOOKUP(M:M,MISSINGUSERNAMEDEPT.xlsx!$1:$1048576,2,0)
you cannot vlookup a column onto another column
also your missing sheet name

your formula should look something more like 
"=VLOOKUP(RC[-3],'[MISSINGUSERNAMEDEPT.xlsx]Sheet1'!C1:C3,2,0)"