The word "Virtua" is in the name of a lot of video games, for example:
I couldn't find a definition online.
I could not find the word virtua either.
Many times, the game makers and webmasters choose the word close to a meaningful word (here, probably virtual). This generally happens as the main phrase as a website URL is not available or the phrase/game virtual tennis is already registered.
It's not observed that every game name conveys a message or have a proper spelling. Game makers and webmasters do enjoy such flexibility.
It's a name, made up on the spur of the moment, probably meant to resemble Virtual. Brand names and trade marks don't have the follow the traditional rules of grammar :)
It's a word that was made up by Sega and applied to many of their early 3D games.