Hi,
I've found the problem.
I don't know if the excel version has something to do with that.
But i'm using portuguese version of Excel and the formula works this way:
ws.Cells[1,1].Formula = "=VLOOKUP(" + tableName + "[[#This Row],[" + ColumnName + "]],sheetName!A:B,2,0)";
I didn't have to use double quotes on string parameters like is said on documentation.
