This is an issue that was introduced with the dragon bones DLC, and still has not been addressed.
In the Clockwork City update, Grim focus and its morphs were changed so that every time you exit combat all stacks will be reset to 0. In VMA, every time you defeat an add wave and portals spawn, you are technically out of combat during those seconds, and thus lose of your grim focus stacks. This does not happen 100% of the time, as in certain stages like stage 8 for example, you are always in combat, due to lava balls being thrown at you. However, in most stages the issue is there, and it is extremely annoying, as you lose a lot of burst damage.
So, what I recommend? It takes 4-5 seconds for portals to spawn, so have the stacks being removed after 5 seconds after exiting combat? A simple change that will make the ability function properly in VMA once again.
Here are the videos, for those interested:
https://youtu.be/w-2SPZ6C34Q
https://youtu.be/elEBGR9d4Ko
Maelstrom Arena - World's First Nightblade 600k Score - 02/18/2017