Maybe a simpler approach to rewriting the whole thing is to just have a macro that you can run anytime you want to RESTORE the original formula to a cell you overtyped the formula?

Here's a macro that will do that. It makes sure your selection is in the column P range, the puts the formula back in. It works for any number of cells you have selected in the range.
Option Explicit

Sub ResetFormula()
Dim Cell As Range

For Each Cell In Selection
    If Not Intersect(Cell, Range("P17:P63")) Is Nothing Then _
        Cell.FormulaR1C1 = _
            "=IF(RC3="""","""",IF(ISERROR(VLOOKUP(RC3,vwlookup,4,FALSE)),IF(RC14="""",0,IF(RC15="""",0,RC14*(1+RC15))),VLOOKUP(RC3,vwlookup,4,FALSE)))"
Next Cell

End Sub