I stumbled upon this, and maybe you want to add this to the manual because I was fighting a bug off for about 3 to 4 hours trying to figure out why one of my panels were not showing up. When you vec_set panels to each other, they work like...

vec_set(panel_1, panel_2); will equal out to be
panel_1.pos_x = panel_2.pos_x;
panel_1.pos_y = panel_2.pos_y;
panel_1.visible = panel_2.visible;

That unknown visible killed me in debugging. Maybe it is in there, but I didn't find reference to it.

I was curious and tried it, but just got a crash.

If vec_set were for panels, I think it would be named pan_set.

vec_set uses the first 3 elements, in your case it used the pos and visible elements. That's why it works with angles ( pan, tilt, roll ), colors (red, green, blue ) and coordinates such as x,y,z

vec_set(panel_1.pos_x, panel_2.pos_x);

Now vec_set have a vector (from pos_x upwards to pos_z), panel_1 without pos_x is not a vector!