A vendor is attached to a product, so if you're uploading products and the vendor is actually changing afterwards this would suggest you have an app (or person) making secondary changes.
If you've just uploading products with a wrong vendor in error it's not too late to go back and bulk edit the vendor or just manually change each.
It's not 100% clear to me the issue you're facing so i've thrown those two comments out above to get things started.
★ I jump on these forums in my free time to help and share some insights. Not looking to be hired, and not looking for work. http://freakdesign.com.au ★