I have the map marker for the vault but I can't find the entrance or sign.
Please help, I've been wandering around for ages.

thank you

It's in a place that is covered in trees, usually has Giant Mantis's in it.

Either by Jacobstown or Black Mountain, I forgot

Pretty sure that's the one that's kind of in a ravine in the middle of a moutain range. If you go back to the flat urban sprawl of Vegas and then start moving towards the marker, you should find it. Has a sign out in front something about plants .