It would be interesting to know the logic and what they think they are achieving.
VDA only provides advisory obstacle protection
until the MDA. Once below the MDA, the VDA is no longer valid -- you are now flying a visual segment and thus must (visually) fly clear of obstacles.
But most pilots don't know this. Most think VDA provides obstacle clearance
to the runway. So with the VDA plugged in, pilots tend to "blindly" follow the glideslope all the way down to the runway threshold. This is dangerous if there are obstacles penetrating the 34:1 (1.68 degree) protection surface.
The example commonly cited is KBHM RNAV (GPS) RWY 36 (Birmingham, AL). If you fly the 3.04 degree VDA to the MDA, then transition to land visually, no problems. But if you continue flying the VDA below the MDA, you get perilously close to a house sitting on top of a ridge, 2nm from the runway. The roof of the house penetrates the 34:1 surface, providing just 190ft clearance below the VDA path (picture below):
House penetrating 34:1 surface @ KBHM RWY 36 (from FAA briefing notes)
So the FAA will no longer publish a VDA if there are obstacles penetrating the 34:1 surface.