Somehow, after finishing with Vault 3, I fast traveled to the Gun Runners and a Ranger met me out there and gave me a two-way emergency radio to contact the Rangers if I was ever in trouble. Unfortunately, I had to go back to a previous save because I had a loading error (I need downloadable content??). I was "liked" at all of the NCR places on the map too. After doing the exact same thing over again, the man doesn't show up and I'm not "liked" but instead "neutral". I can't figure out what I did differently. Any ideas?

Go to McCarran and do the sidequests.

Make sure you're not wearing Faction Armour, that resets your Reputation to Neutral with the faction, my current reputation with the NCR is Liked and I STILL haven't got the Emergency Radio, it seems cool being able to call for Troop support, Supply Drops and even have a Ranger follow you around.

You can have a ranger and a trooper follow you around. It's nice walking around with your own NCR patrol. The supply drops are sometimes empty boxes though....

Will changing out of NCR gear into Vault 13 armor do anything to hurt my rep with NCR?

If you're liked by the NCR, wearing their armor is a bad idea because it sets your rep with them to neutral.

As soon as I changed out of my NCR gear, I became liked at all of the NCR places. Thanks!

Can you use the radio as many times as you want?