An 'unknown' is a person, quality or thing which is not known by most people. 'virtual' is used as an adjective because it describes the noun 'unknown' and is used to mean 'almost the same as' or 'close to being'. So a virtual unknown is someone or something that is almost completely unknown ( but not 100%). Virtually is an adverb we use to emphasize that a statement is almost completely true but the structure would be different He is virtually unknown ( here 'unknown' is an adjective)

virtual = 99% A virtual unknown is something hardly anyone knows about. Example: He knows nothing, He's virtually unemployable. This car is old, It's virtually useless. (It's only use is spare parts)

A virtual unknown can refer to a singer or artist who isn't famous.