400 grit should get a bite from the etch. I wouldn't worry about coating the bevel. At 1000 grit it will etch less dark than the 400 grit. A few passes with a piece of steel wool will make it bright again.
The biggest problem is etching 15N20. The nickel makes etching harder. Vinegar probably isn't sufficient. You can try hot vinegar at 120F and see if it helps. FC would make your life easier.
Make sure the blade is surgically clean before etching. Scrub with soap and hot water. rins with hot water. Wipe dry with a clean paper towel. Etch without touching the bevels.
Using FC:
Make sure the tank is large enough. A 18X3" piece of PVC with a closed flange on one end and a slip cap on the other works great.
Take out of the tank every 3 to 5 minutes, rinse with the garden hose, wipe off lightly with a clean paper towel ( some use 4/0 steel wool gently), rinse again, and put right back in the tank. You have to remove the black sludge from the surface or it acts as a shield against continued etching.
After 20-30 minutes of clean and etch cycles you should have a deep etch (sometimes it takes an hour or so).
Make sure you rinse well and clean the blade, then neutralize the etchant with TSP/Windex/ammonia/etc.
Remember, no matter how long you etch, the etch is only a surface oxide and doesn't really penetrate the steel. Rub hard in cleaning and you will rub it off.