Does anyone know a way to make vector lines thicker in the native visualizer? On a 4K monitor, these single-pixel lines are barely noticeable and look like a complete mockery. Maybe there's a setting somewhere in the system files?

The labs "visualise vector" node can be used to show vectors more clearly and you can set the thickness
overall by pressing "d" in the viewport "geometry tab" wire width.....
(this doesn't work for default visualisers and will change "all" wire widths)

No doubt there is a setting buried somewhere in the UI or a system file as you mentioned for the viewport visualization
vectors but tbh no idea where.

I normally set my background to dark grey and never really had an issue seeing visualizer vectors on 4K as long as I set
the vector colour to something complementary.

This helped me a lot. My eyes send you gigantic gratitude! Great node, too bad I missed it for so long.