Hi SharLynn,
It is not a requirement nor is it absolutely necessary to have your real estate license in order to be a VA to Realtors. In fact, only a handful of REVAs even have their real estate license. Most do not.
There are all sorts of things you can do for Realtors as an unlicensed assistant... Much of the same stuff you can do for any other business. You can help manage their email, manage their incoming leads, maintain or design their websites, help them market their listings with flyers or postcards or on the web... the list goes on.
But there are certain things you CAN NOT do as an unlicensed assistant. Basically it comes down to you cannot act as a licensed sales person by working directly with buyers or sellers, cannot divulge information on listings to the public, cannot do cold calling to try and drum up new business for the agent... things of that nature.
This post here, has some great information that goes state by state.
What is necessary to be a good REVA is a general knowledge of the business, good admin skills, and how to be effective in real estate marketing.