I get out of the vault and I'm standing by the fountain with a hologram that does nothing, my collar is now removed. Where am I supposed to be going?

You are done you should see a gate and when you access it it will ask you if you would like to return to the Mohave. Keep in mind that once you leave you can never get back so make sure you have all the stuff you want from DM because done is done. Congratulations are in order for surviving the SM and for completeing the DLC that most of the ADD types hated. I myself loved it more the second time through, of course i built the character with knowledge that is only accumulated by learning from previous mistakes (my first character barely made it out alive, second blew through it, and i imagine the third character will also as there are some comonalities in builds that make the SM easier to survive.

If interested i will share some of that knowledge or not just ask.

Asai

it's all fine but how do I leave?

First you enlist in your local school, learn your ABC's, then come back here and read the first sentence in the reply to your question.

Go to the gate opposite the one that leads to the casino. Click on it. Gone, baby, gone.