This can vary depending on how the supplier does business, if a supplier sells to consumers (individuals) and retailers like ourselves then most likely you would be classified as a vendor. However, if a supplier only sells wholesale to retailers then you would most likely be a customer to them.
As a rule of thumb, say you are a vendor if you are unsure but always get in touch with the supplier and clarify with them. I would let them know that you are looking to purchase products to resell on Amazon, and they will tell you which one to choose.
What should I classify myself as with on applications?