Populate VLookup Into Blank Cells

spans over columns A:BM.
column headers are in Row 2, data starts in Row 3
the number of rows varies month-to-month
in column A, the values are either N or Y.
Column B contains xREF numbers, where if the value = N, the xREF is unique; for values = Y, the xREF = an N value xREF number (hope that makes sense!)
Where column A value = Y, some row cells are empty. All N record cells are populated.
starting in Column C, I want all empty cells (basically all rows where column A = Y) to be populated with a VLookup formula where:
Ø lookup_value = xREF in column B
Ø table_array = all N values records spanning B:BM

I can get it to work for column C by autofilling to the last row but if I autofill across columns, the col_index_num stays the same and I can’t figure out how to increase it by 1 as it autofills across.

Sub RangeLookUp()
Dim Rg As Range
For x = 3 To Range("A65536").End(xlUp).Row
If Range("A" & x).Value = "N" Then
Set Rg = Range(Range("B3"), Range("BM3").End(xlDown))
ActiveWorkbook.Names.Add Name:="TheRange", RefersToR1C1:=Rg
End If
Next x
' VLookup for blank cells........................