First you have to go to a place called meldon middle school. It's not too far from vault 111. You have to go to the basement in the school follow where the guy goes in the video which then leads to vault 75. There is also a bobble head in the vault. After you clear the vault you have to go to a place called bean town brewery. Follow where the guy goes all the way to the crashed alien ship then follow the green goo on the floor which leads to a cave and then you kill the alien he is relatively weak and then you get your alien blaster.
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I had it activated after getting to the roof of Hubris Comics.