How-to? IF(Calculation Result<>#) Show Dropdown List (Data Validation)

Greetings to all here on the forum;

I have a spreadsheet that uses vlookup to determine the density of multiple materials based on a table that I create as the data admin, however some materials have several possible densities. Thus my goal is something to the effect of a cell that returns a vlookup value unless the material is one of the ones that has multiple possible density values, in which case I want it to show a dropdown list with the allowable densities.

Essentially its an "if normal material, return tabulated density, if not, provide a data validation style drop down list".

You may see attached - this is a simplified representation without any of my macros, hence why it is extremely basic in nature.

Many many thanks in advance for your assistance!