Spoke with Deb Limbert in Vietnam a few moments ago.
Hang Tean was first discovered on the 1994 Vietnam Cave Project expedition. There are effectively two caves separated by a dry valley and the second of these has been explored more recently 2015/16. Each cave is roughly 2.5km long. They do have a vietnamcaves.org website but the recent discoveries have not been updated on it yet. Hang Tean has been incorporated into a new commercial tour run by Oxalis.
Hope that helps