These 2 words are often used interchangeably in the corporate world, with everybody wanting to be a Partner never a Vendor, but they actually have quite a strict difference:
A Vendor
This is someone that’s selling you stuff and just sends you an invoice for it. it doesn’t matter what it is, it doesn’t matter if they give you a discount or freebee stuff, or how large the invoice is, If they send you an invoice and you pay and it’s a simple process like that, then they are a Vendor.
A Partner
Now, a Partner on the other hand is a Vendor that takes on some of the financial load or some of the financial risk for a given project or work. for example, if you are wanting to accelerate a project faster than your current budget would allow but are sure you will get the budget in the next years allocation. Then a vendor might say “look, we’ll give you five extra people for six months. and we won’t be invoicing that until next financial year” then they become a Partner, they have accepted a modicum of risk in the project or given you something that is more than just freebies. e.g. solid internal training that actually costs them something to provide.
That’s it. You don’t call yourself a partner until you’ve actually exposed yourself to some form of limited risk or given the client something that they cannot currently get.
Personal Opinion Addendum
How long does partner status last for? Traditionally corporations are like angels in that they have no memory, but in this case I tend to see this last to the first financial period that the Client company does not owe the Vendor company anything, e.g. if a Vendor forwards a Client a bunch of free consultancy or training at the beginning of a project, but then invoices for that consultancy in the next financial period, they have just used up their partner status, They will still be called a partner but it is unwise for them to assume they can use it until they again show they are willing to resume their investment in the project.
Disclaimer: As always these posts are not aimed at anyone client or employer and are just my personal observations over a lifetime of dealing with both management and frontline associates.