I wouldn't put grease/vasoline on there - it will only attract dirt and allow it to stick. Clean them up and use High Temperature paint.
Warped discs will cause a pulsing sensation when slowing. If it's the front discs I'd expect to also get the pulse feel through the steering. Remember that Rear discs can also warp and your problem could be there.
You can check for warp by clamping something (screwdriver) close to the outside face of the disc, rotating the disc and observe the gap between disc and your clamped pointer. Ideally you would do this with a dial gauge to measure the run-out but I appreciate it's not a tool in everyone's kit.
Warped discs will cause a pulsing sensation when slowing. If it's the front discs I'd expect to also get the pulse feel through the steering. Remember that Rear discs can also warp and your problem could be there.
You can check for warp by clamping something (screwdriver) close to the outside face of the disc, rotating the disc and observe the gap between disc and your clamped pointer. Ideally you would do this with a dial gauge to measure the run-out but I appreciate it's not a tool in everyone's kit.