I joined the Brotherhood of Steel by wiping out the Van Graffs.

Later, I recruit Cassidy and do her quests.

She fingers the Van Graffs and Crimson Caravan as the culprits in the attack of her caravan.

I talk her down from killing both leading ladies to gathering evidence and exposing them to the NCR.

I grab the evidence from the Crimson Caravan, and she's stoked.

I grab the evidence from the previously wiped out Van Graffs and she's like, "WTF, you changed your mind. Now I wanna kill the Crimson Caravaneers, despite the fact that we've gathered both pieces of evidence."

Stuck, can't complete her quest unless I kill the Crimson Caravans, which I don't want to because I like my rep with the NCR.

And I also like Cass.

Any solutions to this predicament?

Well, as I see it you have three different choices.

1. Do Cass's quest.
2, Don't do Cass's quest.
3. Just get rid of Cass.

Have you tried going to Ranger Jackson at Mojave Outpost? That's how I resolved it. I had the same problem, but for an entirely different reason. I killed the Van Graff's because they tried to attack Cass when Jean-Baptiste wanted to attack her before her whole quest started. Ended up having to get the Crimson Caravan evidence afterwards and then drop everything with Ranger Jackson, and even got a chance to loot the ENTIRE store at Silver Rush and made 12000 caps!

funny thing is.. if you stole the weapons while on stealth boy... you karma doesnt drop....

i donno its a some kind of glitch but.. it worked for me...

I didn't lose any Karma when I cleaned out that store either, without a stealth boy. :3

I did get overencumbered though , blame my 4 STR XD

I don't think you lost any karma because their faction is 'evil'.
You gain karma when you kill them, don't you?
I wiped them out with the Brotherhood quest and looted all of the weapons as well, despite Hardin saying he'd send a team to come in afterward to collect 'em.
No karma loss.

You can steal from evil characters.

Thanks, I didn't even know who I was supposed to turn these in to, because since I'd already accomplished part of the quest's objectives ahead of time, I guess Cass skipped that bit of dialogue.
I talked to Ranger Rick or whatever his name was and that cleared up the quest and I was able to finish it.