Alright, after nearly 3 hours the "fast curing" adhesive was not setting (directions said 30-60 minutes); being impatient and also not sure why it wasn't working, I cleaned it all off of the 6800. I had kind of a light-bulb moment while doing this though: VF700 can be mounted in two directions on cards with 4-hole mounts (where the stock cooler has 4 bolts in a square around the GPU), like the 6800 Ultra. So I thought, maybe if it was flipped the other way it'd clear the OEM sink - and it actually does!
I grabbed a few quick pictures of the end result:
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As you can see, some of the VF700's tines are a little bent by the OEM cooler's pins, but the whole thing goes together quite nicely. I didn't have to wrestle with anything - the tines on the top bent out of the way very easily (like with most Zalman sinks, they're pretty easy to flex), and everything else seemed to line-up pretty well. I confirmed contact with the GPU by putting some TIM on, tightening it together, and checking if it was covering the right spot, and the VF700 is properly centering on the GPU (which is correct for the mounting arrangement, but I just wanted to make sure it wasn't being "pushed" out of the way). Kind of frustrating to have wasted the time waiting for the adhesive that didn't work, but I'm happy with the end result here. 😀