Resolved questions
You're welcome in French : Je vous en prie or de rien?

Yes it is!
"je vous en prie" is the formal form but even with a friend you can say "je t'en prie" (but it is less usual).
Another synonym is "avec plaisir" (you are welcome).

[My opinion : It can invit/encourage the person to ask something again to the person (that has said "je vous en prie") because that person was happy to help.]

"De rien" is the common use.

"Je vous en prie" means also that you allow another person to go front to you
(I don't know if I write it well.. is that right in English?). You go after the person in other words.
For instance: "Je vous en prie" when you see a women who wants to go in a shop at the same time as you. You let the place before you enter. you can say that and you can also open the door for the person ;)

I hope it is clear! ^^