Vasculitis experiences?

[WombatStewForTea]

So not to drip feed this is going to be looong.
Not my horse but one I'm 'part loaning' to help out the teenage owner who is unwell and her none horsey family.

5 weeks ago horse went lame on the day I had her. Teenage owner went up the next day and said she looked fine and she didn't see a problem. I went up again next days later and was considerably worse - couldn't miss it. Vet was called (I wasn't there) and diagnosed mites and mudfever. Gave first injection & anti inflammatories and wanted to come back to clip legs and do second injection. Now I've never known a horse to be lame because of mites.

Vet came again 2 weeks later. Legs clipped and vet diagnosed vasculitis. Owners currently debating whether to start steroid treatment and awaiting cushings test first.

Owners have been very vague over everything and young owner has suggested she needs lunging/riding to loose weight and rode her last week and said she was fine. Come my day she asked me to lunge so I did and she was very very obviously lame behind.

I told owners who keep arguing that it's arthritic stiffness (definitely isn't!) Or the vasculitis. Came up today to physically show them but they still can't see it. Definitely isn't just me as another very experienced person has agreed.

Anyway long story short a different vet is coming next week to test for cushings and is now going to be asked to check movement.

I guess my question is this- I've little experience of vasculitis but can it cause what looks like mechanical lameness?