conditional dropdown on intermediate table

I have Vendors, Vendor Parts, and Inventory Parts

Each Vendor Part regardless of which vendor it belongs to, can be related to the same Inventory part. 

For example, Radioshack and Walmart both sell a flashlight with different part#. In my inventory I need it to be the same part# 

I have created the relationship to allow this, however, I am curious how I can go about creating a conditional drop down to select only vendor parts related to a specific vendor and also a specific inventory part
(once a user selects a related vendor, and a related inventory part.)

Can this be done with only two relationships Vendors<Vendor Parts>Inventory Parts  ?

Thanks