Affiliate marketing is completely different from reselling. With affiliate marketing, you market an existing VPN provider and you do not have to build your brand, provide support or manage infrastructure. You just need to promote an existing VPN service and depending on the agreement you will get a commission per sale.
Reselling VPN is done by creating your brand (website, logo, ...) and marketing this brand. The benefits are that you can set your pricing, have higher profit margins and thus can make more money.