From Vanguard Wiki
To solo is to take on alone the challenges a vast gaming world has to offer. A solo player is the image of the lone hero, relying purely on his or her own skills for survival.
While the party is the core adventuring concept to be found in Vanguard, solo and casual (small group) players will be able to adventure and reached maximum level.
However, it is important to note that, as most Vanguard content is constructed for the party encounter, and some for the raid encounter, solo players may not have as rich and varied an adventuring experience as other types of players.