Hi Victoria,
My initial reaction is that it all depends on the services you are offering to your clients. If you are someone that offers web design, then
SEO is probably something you would want to have a good handle on.
If you offer Transcription, then knowing how to utilize all of the software, hardware that goes along with it is imperative.
I would say overall, a very important area training every VA should have is general business management.

Being a Virtual Assistant means being a business owner, so a working knowledge of how to run a business, pay your taxes, do your accounting, get insurance, create policies and procedures, etc....that is a must have and will help you run your business most effectively and professionally.
Business management training can be achieved simply by going to your local library and reading business management books. You can take a course at your local community college or even adult schooling offers general business management courses. Also, if you have access to iTunes, check out iTunes University for some free courses on general business management.
Darlene