I got tired of waiting for Seidio to release a Surface case/holster for the VS980, so I decided to give it a whirl and see if maybe the Seidio Surface Holster would work with a slim case. I decided to try the Diztronic High Gloss Clear Flexible TPU Case, and lo and behold, it fits perfectly! There is absolutely no wiggle/play at all. It slides in easy, as long as you hold the clip back. That's the only "issue" - the TPU material is so "grippy" that the rubber piece under the clip prevents you from pushing the phone past the clip without a lot of friction. To put the phone in one-handed, you have to use your thumb and index finger to pull back the clip while your other fingers guide the phone into position, then the clip springs right back to secure the phone.
So, all in all, I'm super happy that this works. If I were to try to make it any better, I would try a case that was a little slicker to that I could push the phone past the clip more easily.