Only want formula if nothing is inputted, else allow based on data validation

Right now, I have a sheet with a table for vlookup. However, when my other sheet performs the lookup, sometimes I need to change values. But after I make changes, sometimes I want to go back. I am looking for a way to have the vlookup simply fills the cells with "defaults", or somehow only apply the formula if the cell is empty, and even if its deleted, it will recognize the cell is empty and autofill again. All ideas welcome!