=> Create an excel file, say destn.xls with a column which has reference to another excel file say source.xls (Give complete path to the source.xls - something like - "=VLOOKUP(A1,'C:\[Source.XLS]Sheet1'!$A$2:$B$3,2,0)") 
=> Execute the following code:
java.io.FileInputStream fs= new java.io.FileInputStream("C:/test/Destn.xls");
org.apache.poi.ss.usermodel.Workbook wb = org.apache.poi.ss.usermodel.WorkbookFactory.create(fs);
org.apache.poi.ss.usermodel.Sheet sheet = wb.getSheetAt(0);
Row row = sheet.getRow(1);
org.apache.poi.ss.usermodel.Cell cell = row.getCell(1);//assuming this cell has the vlookup formula in your excel file
if(cell != null && cell.getCellType()==Cell.CELL_TYPE_FORMULA)
{
	System.out.println("Cell formula:"+cell.getCellFormula());
}

It gives result as VLOOKUP(A1,'[Source.XLS]Sheet1'!A:B,2,0)
The issue is because org.apache.poi.hssf.record.SupBookRecord.decodeFileName(String) does not seem to be implemented fully