There is good value in learning as much as you can about the industry/ies your clients work in. If your niche is a particular industry this is even more important. Each industry will have a number of things to help educate people in that industry: newsletters, magazines, trade shows, industry association events and the list goes on.
It makes sense, that if you want to provide the best value you can to your client/s in a particular industry, then why not subscribe to their online newsletter? Or pick up a copy of their magazine? Or attend industry association networking events and educationals?
The more you learn about an industry the more value you can add to your client in supporting them as their Virtual Assistant.
Should you be charging them for this additional time? No. This should be considered part of your market research and networking. It’s adding value to your business, just as any marketing and networking does. It’s just that these activities concentrate on a particular industry in order to make you more valuable to your client/s. Why not plan to start learning more about your clients’ industries today?