I wouldn't have it sticking up out of a vice without a thicker piece either side. It will vibrate too much on its own. Better to clamp it to an angle plate.
Or clamp down onto the table overhanging the edge and mill off that way. Or onto a piece of MDF that you can cut into.
Clamp HARD. Really clamp it - you'll be amazed how metal you though you had fixed down can move.
Even though they're called end mills, they're designed to take a rebate that's deeper than it is wide, so don't be afraid to cut on the side.
If you're blanks have been flame cut, beware of nasty hard edges. Sawn or sheared will be better.