I'm having issues colouring the actual line when it comes to vector layers. I know in regular line layers you can just select 'preserve opacity' and colour your line and be happy with it, but with vector layers that 'preserve opacity' option doesn't seem to be available. Any idea? Thanks.
Sorry this is VERY late. But if you are using SAI, there is a simple trick. Create your lines in vector until you are satisfied. Mane a new layer underneath the vector layer, select your vector layer and click "Merge down layer" in your layers box. This will merge the vector with a "normal" layer and prevent you to make any changes to the lines in vector style. Then click "preserve opacity" like with any other normal layer and color the lines.
For Vector lines you have to use the "Color" tool. It's three left from your pen tool when you're on your vector layer. If you look at the last screen shot in the tutorial, it's right under the color blocks.
Click Color, select the color you want, and click the line you want to change.