There are a number of issues relating to the ways in which external services such as HERON can be embedded in the VLE. (The HERON example is interesting because of copyright clearance issues, but the issue is more generic and relates to a variety of potential services.)
For instance, it is still common for some services to be restricted by university IP address rather than, for example, Athens-enabled so that off-campus use is not enabled. Where services are (relatively) standalone this can be made clear to users but it is more difficult to handle this cleanly once they are embedded in the VLE.
At the object level, it appears that while some publishers are happy to grant permission for, say, an image to be used in classroom teaching they are not happy to allow it to be delivered in a VLE, regarding this as 're-publishing'. If this cannot be handled seamlessly (from academic staff and student viewpoints) it could undermine the use of VLEs.