Well, on a PC you can pretty much set your rad tolerance/resistance to being immune to the radiation. However on the Xbox 360 or PS3 you need to have 85% rad resistance and a heck of a lot of rad-away. This is because you'll have to run in and discover, then run out. Like most others vaults, the entrance looks like a wooden door; but this little door is [INACCESSIBLE]. However, it is possible to discover it. All you have to do is wear an Advanced Radiation Suit, give yourself a dose of Rad-X, and hot-key your Rad-Away. That way you can keep running while giving your self Rad-Away every 2-3 seconds. Right as the games says you have discovered Vault 87, quick travel out of there. Note that this does take away around 75 of your Rad-Away, so wait until you have at least 150 Rad-Aways before going to discover this area.