So I decided to look into the faster vulcan and found a few things:
It seems that the faster vulcan trick from Metal Slug Advance doesn't carry over entirely to Metal Slug 7. In Metal Slug Advance, pressing B on the earliest frame when the slug fires 5 more shots will automatically overlap the 5th and 6th bullet. However, in Metal Slug 7, if you press B on the earliest frame where the slug will fire 5 more additional bullets, there will be a noticeable gap between the 5th and 6th bullet as seen below:
However, if you press on a frame where the slug fires 4 bullets, the gap will be filled as seen below:
So, it seems that the fastest way to shoot is to press B such that the slug fires 4 additional shots at a time after the initial 5 shots.
Interestingly, the faster H still works in Metal Slug 7, which makes me think the faster vulcan should also work, but I haven't been able to do it: