VDT, or View, Download and Transmit, is a set of requirements for 2014 Certified Electronic Health records under Meaningful Use for the EHR to enable the patient to View, Download or Transmit their data. In addition to VDT requirements, the EHR must also use the Consolidated CDA (CCDA) under those regulations, and must support The Direct Protocol, and may support other protocols.
Providers who want to get incentive $$$ for Meaningful Use must use the standard format (CCDA) when enabling patients to Download or Transmit data. They may, but are not required to use the standard protocols the EHR must support for transmission of CCDA documents for referral. This is a rare exception to the general rule that to obtain the incentive, you must use the certified capability. In making this exception CMS understood that some very functional referral networks already had document transmission capabilities and did NOT want to have to replace them with The Direct Protocol just to support the same feature.
Blue Button Plus is an extension of Blue Button that uses the CCD and CCDA formats specified under the 2011 and 2014 Certification and Standards rules to exchange patient data. When the document is transmitted using The Direct Protocol, effectively you have the BB+ Push capability. When it can be downloaded using a pre-draft subset of the HL7 FHIR standard, you have the BB+ Pull capability.
Neither the 2011, 2014, or proposed 2015 certification criteria require Blue Button or Blue Button Plus. Blue Button Plus has been proposed for 2017 certification criteria.
Oh, and finally, Blue Button, Blue Button Plus and Open Notes are very similar. When you can get your Open Notes using the CCD or CCDA format, effectively you are using the same content standards supported by BB+, and then it is all a matter of how you would download or transmit it.
That is the challenge when setting the low bar in VDT. More functional bars like BB+ Push or BB+ Pull aren't required by Meaningful Use, but are perfectly capable of meeting the requirements for it.