In this case: I usually go to the top view, click and wiggle the last vertex a bit, and then click 'undo' (to place that vertex back to its original location). Then switch to a side view and start adding vertices to that. I don't know if this is the correct way technically, but when you enter the side view after that, your new vertices will be added to that plane. Basically, whenever you add a new vertex in a 2D plane in NL2, the depth is going to be placed at default based on your last action in the depth plane.
^ This is my solution too, this was also a problem in NL1 if I remember correctly. It's not a huge thing as long as you're not like me and forget about it every single time
^ This is my solution too, this was also a problem in NL1 if I remember correctly. It's not a huge thing as long as you're not like me and forget about it every single time