I mean, with guns, you have silenced weapons. I've never focused on energy weapons, but I seem to never see an energy weapon with "silenced" on its description

I don't think you can, unless you can do it from far and wait it out until you're hidden again.

The Project Nevada weapons pack has my stealth laser that I ported from FO3 for them. Silent shot, silent nearly UV beam, pretty nice damage.

http://newvegas.nexusmods.com/mods/images/40040-1-1318635417.jpg http://newvegas.nexusmods.com/mods/images/40040-2-1318635420.jpg