Automatically copy cell if blank with vba

Here's a non-VBA way:

1) Highlight Column A
2) Select all blank cells (Ctrl-G should get you to the "Go To" box, then click "Special", click "Blanks", and "Okay")
3) All blank cells should be highlighted
4) Type "=" and then arrow to the right of the blank box (so if the first blank box was A1, you would now have "=B1") (don't click with the mouse anywhere otherwise all the blanks will be unselected)
5) Press Ctrl + Enter
6) If done correctly, it'll fill all blank cells with the contents of the adjacent cell in column B

I'm sure someone has a better way, but I have found this useful many times. Hope this works for you.

Sub With_Loop()
Dim c As Range, lr As Long
lr = Cells(Rows.Count, 1).End(xlUp).Row
If WorksheetFunction.CountA(Range("A:A")) = lr Then MsgBox "No empty cells in column A!": Exit Sub
    For Each c In Range("A2:A" & lr).SpecialCells(4)
        c.Value = c.Offset(, 1).Value
    Next c
End Sub

Following leaves a formula in the previous empty cells.

Sub With_Formula()
Dim lr As Long
lr = Cells(Rows.Count, 1).End(xlUp).Row
If WorksheetFunction.CountA(Range("A:A")) = lr Then MsgBox "No empty cells in column A!": Exit Sub
With Range("A2:A" & lr).SpecialCells(4)
    .Formula = "=RC[1]"
End With
End Sub

If you look at the cells that were empty before you ran the code, you'll see that there is a formula in it.
All the code does is put a formula in the empty cell.