vFG worked fine for my group yesterday (NA/PC). Which megaserver (EU or NA), and are you on consoles or PC/mac? It may be a bug in the console version that isn't there for PC users. Videos would help the devs a lot if you can capture and post an example of the issue.

Note: especially in Vet mode, there are a lot of bosses with special mechanics, where they are sometimes invincible until you do something specific. This is part of what makes the vet dungeons MUCH more challenging than normal mode. Examples:
1. in (normal) Blessed Crucible, the final boss (Lava Queen) is shielded when the flame atronach adds are up - you have to kill those to do any further damage to her.
2. in Vet Darkshade Caverns, the Transmuted Kwama Warrior (2nd boss) occasionally gets a big damage shield and does a huge ground pound AOE. You have to DPS through the damage shield to end the effect.
3. Keeper Imiril in Vet Banished Cells (2nd boss) occasionally disappears and has adds appear instead, for a while, then reappears on a timer. There are similar boss mechanics in several dungeons, including the new Imperial City Prison (final boss), and White Gold Tower (final boss).
4. Second boss in vet Fungal Grotto has several nasty mechanics, including disappearing and summoning four shades (which you have to kill before you fight her again), and sometimes just disappearing for an extended period, but then reappearing and continuing the fight. (There are other challenging mechanics too).