While I use different software, if I needed both raster and vector elements in a piece I'd go about it like this:

Do the traditional illustration work first, and scan it into the computer at high resolution as stated in your post.
Next, I'd do any raster artwork you need in GIMP. Export the image from GIMP, again, at high resolution.
Import the artwork into Inkscape and do your vector work.
Export the artwork as a PDF from Inkscape, and any vector work you have should remain vector.

I hope this is useful.

I think it's fine. As long as you're not selling it, you can do what you like. I'd like to get round to doing something like this myself with some classic fairy tales.

As well as the great points raised about the lips above, I think overall you could improve this by thinking more about the position of your light source.

This is beautiful work, it has a timeless quality to it. I'd love to see how it looks after you've dropped a couple of gradient maps on it to colour the values.

Something like Goolge SketchUp might be all you need. It's pretty basic, pretty easy, and pretty free.

Welcome to the forum! Loving your work, really nice stuff.

That's a great spot, thanks @RobinCampbellArt. He was intended to be shown looking through the bottle at some new marvel he'd collected or created, but I see now his eyes need to move a smidge to the left to be lined up convincingly. Thanks.

Loving the colours, loving the change to make the pig waving now, and overall looking really nice so far.

@Aleksey I love how the frog is in such a spider-man pose, yet still works as its own thing too. Would love to see him trying to swing clumsily around New York using his tongue instead of webs. Maybe he's a mutant that got accidentally brainwashed to think he's Peter Parker?

@hannahmccaffery thanks very much!

@Gorillo the client trains in martial arts so the dwarf needed to look powerful as well. And yeah the contrast in detail was intended to make the character stand out, but if it’s gone so far as to be jarring perhaps there’s a better balance to be found.

Thanks for the awesome feedback so far everyone, this is great!