lookup of a barcode and if present load value from one list and if not from another

Hi,
I am scanning a 12000 tubes some of which are in a keep list and some are not. The ones in the keep list I would like to assign the next sequential position in a keep boxes whilst the ones that arn't in the keep list I would like to assign the next position in the dispose boxes.

Can anyone help me with how I can get excel to do this live.
I can scan in a barcode and get it to populates the next cell down.
I can have a vlookup in the cell next to it which will return matching data from the keep list if it is in the Keep list or an "#N/A" if it isnt in the keep list
I can use IFERROR()to reassign the #N/A error a value (Dispose)
I have generated a list of all coordinates in keep boxes and in dispose boxes.

What I don't know how to do is to get excel to assign a barcode the next sequential position in the keep boxes if it finds a vlookup match or assign the next sequential position in the dispose box list if the vlookup comes back as #N/A (Dispose after IFERROR().

Please see attached example of where I am so far
Many thanks for any suggestions

Matt