In brief I would say that PLEs are mostly used for informal learning whereas VLEs, due to them being implemented by educational institutions, will inevitably focus on formal learning.
How can an institution formally endorse a PLE? I would say not possible, if not in small courses.
An institution, in particular courses, might recommend / guide students to choose specific (type of) tools through which activities/learning may be done.
In drawing together my PLE I have highlighted the use of a web browser. In fact the browser together with my PDA, laptop and PC are the main blocks on which my learning occurs. I depicted (two) institutional VLEs that I use as forming part of my PLE. Moreover my preference in using a number Google applications has evolved due to the use of the same username (identity) for different apps.
It will be interesting to see how Google will go about in developing their OS, which is going to focus on web interactivity. This one tool which might revolutionise the PLE concept.