While Vincent was in Quinn's body, using the shadow abilities still felt very foreign to him. He could use the blood skills in Quinn's body without having to rely on the system because he had used these skills and the same moves in his body a long time ago, again and again.
At the same time, Quinn's body, due to it having evolved into the Vampire lord stage, wasn't actually all that different to what Vincent had in the past. But the shadow abilities were a different matter altogether. To use them, Vincent had to access the system. When using the shadow link skill, it gave him a few options of people he could connect to that also had their shadow link on.