I personally am choosing to not make "virtual assistant" a necessary or integral part of my business or marketing at all, because my target clients don't know what it is and won't go looking for it.

My new business cards have my name, then "Your Personal Assistant" as the title. My business name at the top is "Your Office People" and the subtitle is "On Call Administrative Services". Instead of a physical address I listed industry specific services for my original target clients (local contractors). I think it conveyed what I wanted for this first marketing effort.

I may make use of Virtual and Virtual Assistant on my website and cards for different industries, but I am really preferring "on call" to either virtual or outsourced