Look on wordpress.org for the plugin called “Groups”, and then from WooThemes.com look for the Groups for WooCommerce addon for it. You would need BOTH of these.
1.) Install both plugins and activate them
2.) Create a new group in the Groups setting called “Paid Vendors” or whatever you like.
3.) Create a WooCommerce product, called “Buy your vendor membership” or whatever, set a price, and configure the product so that it adds the buyer to the group “Paid Vendors” for X days or X months or whatever you like.
4.) Restrict access to the Vendor Dashboard pages to the group “Paid Vendors”.
When the buyer buys the membership, they can then see the vendor dashboard. When a buyers membership expires, they can not see it anymore unless they buy their membership again/renew it/whatever.