As seen on the Nuendo forum, using VCA Link prevents the audio tracks volume automation from being sample accurate :
It seems that it (the sample accurate automation) has only been implemented for audio tracks (Audio, Group, FX and Instrument) and not for the VCA, and since the VCA position is always being read in real time before the audio track fader, it will send its own “automation clock” (because of the Link being active, it is the VCA that first takes control the audio track fader, even if the VCA isn’t automated).
That’s a huge design flaw actually, VCA should be sample accurate too.