through little lamplight. its in one of the main quests. when you have to find the GECK
yes thats the only way. you cant go through the front area as it s heavilyradiated which will kill you when you get close. btw when you decide to go there bring a lot of ammo and stimacks. its the most heavily concentrated area of super mutants moreso than the mall. good luck.
Edit: Actually, it is not due to the radiation that you can not get to Vault 87 using the front door, but because it is unable to open. The door is damaged and can not be open. Reaching it from the front is only useful to gain the caps that Raily gives you everytime you discover a new location, which you'll have already spent stockpiling radaway.