My mom, 60+ year old, decided she wanted to learn proper English. Being an aircraft controller, she already knew enough to get by on the job (communication is pretty much standardized). But nowhere near enough to get by in the US.
After a few conventional approaches (such as physical English classes), she somehow found out about Second Life and enrolled in a (paid!) course.
Fast forward a couple of years. She can now hold conversations in English and go out by herself without a family member translating. As an observer, the progress was outstanding.
One thing that I have noticed is that it can be way more interactive than "normal" classes. For instance, there was a lesson that took place at an airport. So, instead of just an opening dialog in a book, and then some discussion, their avatars were at a functioning airport. So they could go all the way from arriving at the airport to boarding the plane, having dialogue all the way. Not sure if the airplane did or could take off though.
At another one that I witnessed, they were at a clothing store. So they could actually interact with a human shopkeeper, buy the article of clothing they wanted, then put on their avatar.
Having attended "standard" classes all my life, I was blown away by how much the virtual environment would foster dialogue. Usually, a teacher has a hard time coaxing this kind of interaction in a classroom setting.
Also of importance is that, since you are hidden behind an avatar, shy people seem more comfortable. Things such as race and, to an extent, age, are completely hidden.
Not sure if VR would help anything, except for the "cool" factor, which can't be discounted.