Welcome to Rust, I hope you enjoyed learning and using it :)

One major advice would be to run cargo clippy, which gives a lot of helpful hints. Apart from that I also have some feedback.

    
    40	struct VendorOuiData {
    41	    vendor_organization: String, // Vendor Organization

  

These comments should be doc-comments (///) so that cargo doc picks them up.

    
    49	fn fetch_oui_database(url_string: &String) -> Result<String, Box<dyn std::error::Error>> {

  

Not saying that your return type is bad, but in general you might be interested in crates thiserror and anyhow that simplify error handling.

    
    51	        .timeout(Duration::new(60, 0))

  

Duration::from_mins(1) would be much more readable.

    
    66	                panic!("{}", err);

  

You should almost never panic. Your function already returns a Result<>, so you should be using that for error handling.

    
    70	    } else {
    71	        // HTTP Status is not 200

  

Not Rust specific, but usually I recommend to handle errors first, and return from the function early. This way you reduce cognitive load on the reader, and reduce nesting of if-blocks.

    
   150	                let mut parts = line.splitn(2, '\t');

  

There's a convenient method to split in two substrings that you could have used:

    
let (mac_address, vendor_organization) = line.split_once('\t').unwrap_or(("", ""));

  

    
   166	                vendor_oui_data.vendor_address += line
   167	                    .trim()
   168	                    .split(' ')
   169	                    .filter(|s| !s.is_empty())
   170	                    .collect::<Vec<_>>()
   171	                    .join(" ")
   172	                    .as_str();

  

I would probably write a regular expression that replaces all contiguous whitespace with a single space instead.

    
   173	                vendor_oui_data.vendor_address.push('\n');

  

Aren't you trimming this new line off in line 181?

    
   181	        vendor_oui_data.vendor_address = vendor_oui_data.vendor_address.trim().to_string();

  

This is somewhat inefficient, since you'll be allocating and copying your string here.