As long as you mention the creator of the elements that you are using, and include a link to the artist, it is fine to use these elements for your entry of this contest. If your design is a winning design, we are going to use the idea instead and re-create it. But for posting on Hive itself, you are required to have a commercial license in order to be able to publish it here.

In short; if Vecteezy offers a license where you can use the work commercially with a mention then it is okay to use them for your entry.