Thx Tomo, yes I did this eventually.
Vault 15 is unguarded but better than nothing.
I understand now why the super stimpacks are sold so expensive on the forum.
PS : nothing to do with my previous post but i just discovered something.
Apparently, if you travel in the wasteland, poisoned and reach 0 hp, then, you die straight away.
But something strange happened.
I was hunting bp and exploded on a mine, i had -1 hp.
One rat arrived and hit me, -3 and I suddenly died... I can not explain that one.