In what circumstances do you prefer vertical mattress sutures?

I use them for tunnel grafting whether I use connective tissue, alloderm, or combinations with PRF. You use them to position the native gingiva vertically to suspend it over interproximal space bonded together to hike the suture higher for maintenance of the overlying tissue position during healing of the graft.

As a perio resident I would use them as Jennifer suggests. Now 20 years later I never use them because I have found that any oral/perio surgical procedure can be successfully closed utilizing interrupted Chromic Gut suture

I've found that when I advance a flap for GBR, releasing the periosteum that vertical mattress sutures are invaluable especially when covering TiMesh, PTFE membranes.