Hi Simon, Yes, a vise is helpful. In fact, if you are just holding your practice plate on a table and engraving, that makes it tougher. What will happen is the plate will move the smallest amount during impact jolts and power is lost a lot so that it takes more umph to cut. So with that, for now if you are without a ball vise you could just clamp your practice plates in a bench vise.
You may have found it already but here is a link to short practice exercises.
https://engravingschool.com/private/tut1.htm
PS..
I am noticing the finish on your practice plate. Very nice job with the finish you have on it!