I got you bro, since QB employees like to copy a single word from your subject/title and paste the top result from the forum... >.>

1. In the top bar of Quickbooks, click the "Lists" dropdown menu, then select Add/Edit Multiple List Entries. 
2. Select the List dropdown arrow and select "Vendor Items".
3. Select the Vendor dropdown arrow and select whichever vendor you'd like to add unique part numbers for.
4. Under the "Item Name" column, enter the "generic" item name (also called shop numbers, etc).
5. Under the "Vendor Part Number" column, enter the unique, vendor-specific part number.
6. Save changes when finished.

***Note: For instance, I buy an identical product from 3 vendors and we know this product as a "TA50". That is our generic part number. Vendor A's part number for this part is 5140108, Vendor B's part number is VTA50, and Vendor C's part number is ARFSB50.

To create the correct vendor-specific part numbers for each item in this scenario, I would select Vendor A from the Vendor dropdown, type the generic part number in the "item name" column (TA50), and type the vendor-specific part number in the "Vendor Part Number" column, on the same line. You can also type "Alternate" in the "Vendor Type" field if you'd like that vendor listed as an alternate vendor for the specified item. Do the same with Vendor B and C.***

Too bad we can't use this field on POs...

When I type in the Item Name (internal name like your TA50) on the 2nd vendor, I get an error that says Duplicate Entry.
I still do not know how to do this.

This is the most SIMPLE thing that should be basic programming.
How can QB assume that a raw material that goes into an assembly will have the exact same PN for several different vendors???? If I make a separate ITEM for each of the vendors, exactly HOW will I make an assembly? 

Item REDSCREW (inventoried part) is sourced from 2 different vendors. Vendor 1 calls it REDSCREW000, Vendor 2 calls it REDSCREW999. Item REDSCREW goes into assembly REDTOY. I need to be able to buy the red screws from whomever has them in stock the cheapest. When they arrive into inventory, I need them to be used to make the redtoy. I don't care who sent them.
You are telling me I have to make 2 SEPARATE inventory items for this same screw???? Then how do I make the assembly? I can't call out both screws in the assembled item! 
There is no place in the assembly item where I can call out Alternate items in case one is out of stock....so that won't work.

I am in this EXACT same boat. I sell chemicals, and my assemblies' part items (raw materials) need to be able to come from different vendors. What is the freakin point of the alternate vendors function if we cannot use a unique part number/description/tradename for that vendor's product!? I just need to be able to stock 1 inventory item from several vendors, all of who sell their products under different tradenames. This seems like it should be a basic function for QuickBooks...

I understand the significance of being able to assign unique part numbers for multiple vendors using the alternate vendors function, wheatbarley36. Allow me to share a workaround for you.

You can utilize the class tracking feature in QuickBooks to organize your transactions by tradenames. First, let's ensure it's enabled from the Edit menu:

1. Navigate to the Edit menu, then Preferences.
2. Select the Accounting tab, then Company Preferences.
3. Turn on the Use class tracking for transactions option.
4. You can also switch on the Prompt to assign classes function, which will remind you to select a Class when generating your transactions (optional).
5. Once done, hit OK.
   

Next, create a class for each vendor's tradenames:

1. Go to the Lists menu, then Class List.
2. Click the Class dropdown at the bottom of the page, then New.
3. Enter the vendor's tradename as Class Name.
4. Once done, hit OK.
   

After that, you can assign the tradename when generating a vendor transaction from the Class field:
 

On the other hand, you can leave the Manufacturer Part Number (MPN) blank when enabling this feature, as suggested by my colleague above. Then, manually generate the MPN when creating a transaction and use the Description column to populate the specification for the item:

Additionally, you can pull up and customize your vendor reports to get a snapshot of your upcoming expense totals in the software.

If you have more questions about managing the alternate vendor function or generating part numbers, hit the Reply button. Our team is always available 24/7 to help you.