Need 1 cell to be manual & calc'd

This might be odd, but is there a way to have one cell store a formula but also let users manually override formula when it doesn't meet the criteria (without erasing the formula)?

I have a worksheet that users fill out. There are approx 100 cells to fill out and all user cells are in one column (there is protection on this sheet).

There is one question that has 4 parts. Users select a "vendor" from a drop down list (vendor table is on another worksheet) and if found, then 3 other pieces of info is pulled in from the table (using vlookups).

The problem is when the vendor is NOT on the list. Users must then manually type in the vendor name plus the 3 other pieces of info required - but doing this erases the vlookup formulas.

I'm not real vb savvy, but I tried using Worksheet_Calculate to get around this - unfortunately this gets triggered when anything is updated on the worksheet (way too slow!!).

Any ideas how to get around this?

Hopefully someone can add a better way around this, but I just create a cell next to the value to override.

So the cell with the vlookup changes to:

=if(overrideCell = "", vlookup, overrideCell)

Hi, thanks for the idea, however the format of the worksheet needs to remain unchanged - all the user input cells are in one column and the columns before and after are already occupied.

If there is no way to use vba code in the background to handle this, then I guess I will need to remove this section from the worksheet.