I don't see any difference between relational aesthetics and the phenomenologies that were established by minimalism in the 1960s. They are both concerned with the "condition" that establishes "the work."
To say, as Bourriaud does, that relational art works "take as their theoretical and practical point of departure the whole of human relations and their social context, rather than an independent and private space," is just an extension of the in-your-face environmentalism of Donald Judd or Richard Serra. Throw a Jenny Holzer slogan in there and you've got postmodernism. Rehash it as a "relationship" between viewer and art condition, and you're just restating the obvious.
Relational aesthetics are a way of defining an art condition, with special emphasis on its relation to social and physical space. It is a field that was well-tilled by the minimalists. Immersionism is also a development of the early 90s, and it is also involves interactive and relational aesthetic experience. However, these are not necessary conditions for the immersive event. One has found in immersive events quite conventional, stand-alone art works, as well as interactive works. There are large communal spaces, and frequently as well cloister-like private spaces.
This is because in Immersionism the "theoretical point of departure" does not reside in the question of "human relations and their social context," a piece of jargon borrowed from the social sciences by art criticism. Hardly the first time this has been done.
Immersionism leaves human relations alone, does not try to inscribe "human relations" into aesthetics. One notices in the immersive aesthetics, just to cite two examples, of Nerve Circle and the Federation of Ongolia, that the aesthetic condition is already resolved. It does not hang self-consciously upon the entrance of a "viewer" to complete its condition. The world is already made, and the viewer is immersed in it. In what aspect the viewer or the participant stands in relation to the immersive condition is not really a concern of Immersionist theory.
Relational aesthetics advance a pedagogy of participation in which the participant completes the event. This is barely any different from the essentialism explored nearly half a century ago in, for example, a row of bricks on the floor by Carl Andre. Immersionism advances a world without these self-conscious comforts. The participant has to sink or swim, both as a "subject" and as a "communal" member.
Immersionism starts with systems that engage loops and spheres of information and sensory feedback. And these systems lie beyond just the sphere of "human relations." If Immersionism has a godfather from the 1960s, it would probably be Buckminster Fuller, rather than the minimalist and conceptualist art world.
Relational aesthetics entail a program about the use of gallery space and the "art condition." Immersionism addresses aesthetic experience as a general matter, and immersive practice has rarely taken place in galleries or theaters.
General Introduction to Immersionism
Compilation of links to Facebook albums on Immersionism and North Brooklyn art history
Facebook album on Immersionism in Williamsburg