I had c.diff a long time ago, and don't remember the details, but my brother just had it this summer. Yes, after an initial round of vanco, he did get bad D again, and yes, his stool tests were negative 3 different times, but the D continued. So his Dr. put him on the Vanco anyway, again. But this time he tapered the medication for over an 8? week length of time (or 10 wk, something long like that). That worked. It's never come back! It's been 2 mos.
Could it be the vanco is wiping out your other good bacteria? Would probiotics help? We tried to get him to take S. Boulaardi. He never did.. ..But my research showed that might be a good one for c diff. I had read VSL#3 worked for some, but made others with IBD flare.
Anyway, his doc said the tapering vanco is the newest research, most successful way to go. Also, heard there's a lab that tests c diff in your urine (I think, or is it blood, no, I think it's urine), lab is Metametrix I think. Which is a good way to test, I've heard, as the stool test is way too random. If there's no c diff bacteria in that small smear sample, it's a false test! Best luck-