IF and VLOOKUP formulas

I have a invoice worksheet and product pricing worksheet. In the Invoice worksheet I got the Item #, Description, Unit Price ,and Extended Price columns.I want to know how to do a IF and VLOOKUP formula to test for an empty cell in Item #. If the cell is empty, The description,Unit Price, and Extended Price cells are left blank. The VLOOKUP function uses the item number to obtain the appropriate description and unit price form the Product Pricing worksheet. I also have to place the appropriate formulas in the Description, Unit Price, and Extended Price cells.