I would suggest diving a bit deeper into the HOW you wish to promote an event or services and then make that the niche.
For example, if you want to promote books for authors decide on the services you will offer to make that happen.
Authors would be your target market so you would find the areas they will be located.
Offer to promote their books via social network marketing, article marketing, through book reviews, assisting them with writing their newsletters, offer to set up an autoresponder program so they can capture email addresses and so on. This would be the HOW.
All of this falls under the umbrella term 'marketing' but that can have more than one avenue. There is online marketing, print marketing, ad marketing, social network marketing and so on.
Hone in on the skills you have and then use those to promote authors' books and publications.
As for your question regarding VAClassroom, there have been numerous threads about this here on VAF. Craig Cannings has been a guest on the live chats. Here's that
link.
Janine