Creating IF Function For Blank Cells

first post, i'll try to make this short and to the point. i have a file that has single invoice numbers, invoice dates and invoice totals. the line does not have a vendor name. the vendor name is above the invoice lines. (see spreadsheet).

Example:
D2 = Vendor
Lines 3-10 are individual invoices

Basically, I need to setup a formula that says if a cell in Column D is blank, then select the closest cell above the blank cell. I have 65000 lines with thousands of vendors. In the end, each invoice line will now have the vendor name that it is associated with.