I try to talk to Cato, but every time I try to move near( not even close to him ), I was spotted by ranger.

I was wearing NCR trooper armor, using stealth boy, I had silent running perk and I only move when ranger( the one who patrolling around visitor hall entrance ) is facing back. Other rangers were quite far from me...at least those who I saw, then who is the one spotted me and how he spotted me?

I try to kill the ranger who patrolling by using sniper rifle with silence mod from very far distance, but still I was spotted.

Am I must talk to Cato or else Kimball will not come?

The dogs can sniff you out, and if i remember they have a higher detection than the rangers.

personally the easiest way to do this mission is really just set the flak cannon to shoot down the vertibird and stealth boy in.

Now, if you are having way to much trouble getting to cato, or you figure its a bug, you can try killing off the dogs and/or the rangers with the console (if your on the PC)

simply hit the ~ key (located to the left of the 1 key) and click on your target NPC, then type kill

if your on the Xbox or PS3....well i dont know to be honest

Try using the NCR Face Wrap armor with some authority glasses and maybe 1st recon beret. There are multiple ways to kill Kimball it's crazy. At the dam itself, there is a snipers tower to climb on, kill the ranger and snipe Kimball or use the Euclid's C-Finder. You can rig an AA gun to blow, and if you can't rig the vertibird(you need to talk to Cato), you could talk to a guy who lost his helmet, and rig his helmet to blow. Those are the things I do. You can also use a device, I think it was a radio, to screw with the vertibird and force it to crash whenever it takes off.

Take on of these solutions I do, find which has less dogs/rangers and go with it.

If you put on a stealth boy you can just sneak by.

The way I did it:

C4
Ground
Detonator
A good spot, far enough away to not get seen, close enough to see

Result: Rain in form of Kimball, some great stuff really.

I've always wondered how to finish this quest without being vilified by the NCR.

Actually, scratch the quest because I don't like the Legion. I love my Meat of Champions Perk. While I do lament killing another Elvis lookalike, killing House and Caesar doesn't bother me.

Problem is the Remnants and the fact that the NCR owns over a third of the game, and some nifty areas too. I can silently kill Kimball and get away with it correct?

Sorry I'm piggybacking your topic, but I'm in no mood to search for a similar topic or make a new one.

You get hated, even if you are unseen.

The best part of Kimballs death is his auit. My favourite suit, followed by the Ranger Vest Outfit.

To Ildun, the same thing happened to me. I think that my solution was to avoid him, and walk the long way, or it could have been to go past him when his back is turned.

And the way I killed him was rather unimaginative. I just hid next to the door he comes out of and hit him.