If you mean oakleyvault.com then yes it is legit, its the outlet store for Oakley. Just like they have O-Stores and Vaults as retail locations, they have oakley.com and oakleyvault.com online.
To find a site is legit or not, you can do some methods. You can check whois lookup for a domain name. If the information is public and looks legit,you can note the email id, contact number of the domain name owner or administrative or technical department or else sale department. It will help you to contact them if you have any problem after using their services or buying their products.