Excel: I need a cell to use VLOOKUP to find a value and display it, but then allow the user to choose a different value from a drop down list

like I said, I need a cell to display the result of a VLOOKUP and then allow the user to select a different value from a dropdown list, if they choose too. I used data validation list to create a dropdown with all of the possible values a user could want. Based on the input from a separate cell, I want to use VLOOKUP to find the value I want and display it. But if the user doesn't like the resulting value, they need to be able to change it by picking a different value from the dropdown list.

When I create the dropdown full of all possible values, I cannot enter a formula overtop of the list. When I try I get a message that says "The value you entered is not valid. The user has restricted values that can be entered into this cell."

So then I tried putting the formula in the cell before I used data validation to create the dropdown. This worked the first time I entered a value into the criteria cell. I was then able to select a different value from the list. Unfortunately this only worked once. After I selected a different value in the list the formula would never update when the criteria cell for the VLOOKUP was changed.

Can anybody help me accomplish this? Thanks!