I'm managing to keep Ripley's coat quite nice by giving a quick brush everyday and a good long brush once a week, but the fur around her anus takes a lot of work everyday to keep clean even though she does nice solid stools. My question is, could I trim her the fur around her anus and vulva  or would it look weird/ruin her coat?
Also any tips on keeping her shiny in that area if trimming is a no no?

Depends if you show her.

She's a rehome, I only took her on a couple of months ago and she didn't come with any papers so no showing for her.

If you're not showing you can do whatever you like with your dog's coat. I would use thinning scissors to trim under the tail so you can shorten it to keep it clean but still leave it looking fairly natural.

I know exactly what you mean, we had to trim lots of fur off ours, they are hard to keep clean and mat free, the thinning scissors is a good idea, i would think as long as it looks okay from the rear end it would be okay, if showing would a judge check the coat there?

I don't know much about Shelties, but I would guess a judge would generally be checking coat quantity and quality, and if the trimming was obvious that would mark you down, also even if not obvious but making that area look sparse that could mark you down. I'm not sure though! I doubt it would matter just at companion shows, judges have to judge dogs in all sorts of pet trims, so doubt a little sanitary trim would matter there.