Just some little bits of info.
- You were running a benchmarking tool at first, which is something that literally pushes your card to its physical limit. However, as gman450 stated, 99 degrees is WAY too hot for a 280x
- Running with vsync caps your framerate to your monitor's refresh rate to prevent tearing. More frames obviously mean more processing, and in turn that means a hotter card. Hence, as a rule of thumb and if tearing isn't so visible it hurts, you usually keep vsync on if your gpu can handle the game at any fps value higher than your monitor's refresh rate at all times, and otherwise keep it off. There really is no advantage in running a game uncapped otherwise, unless you want to benchmark it. It'll just put more strain on the gpu.
- As gman450 suggested, keep temps under control in any case (e.g. I'm paranoid about my 290x, so I keep gpu-z opened everytime I fire up a game). A 280x should reach AT MOST 70-75 degrees under common gaming load, unless there are airflow issues or heat sink issues.