Hi, I don't know what to do!!!! Please, this is the bug:

During the quest "You Can Depend On Me", I am sent to deliver an invoice to Dr. Hildern at Camp McCarran, but when I get there I can only say "Goodbye" or "See You Later". This bug doesn't let me complete this quest and "Heartache By The Number" and I don't want to end the game leaving questions behind. I mean, I have seen lots of bugs in this game but this is just too much.

Can anyone tell me what I can do to overcome this problem???

I had that same glitch on my first playthrough. It's unfixable. You can't continue the quest, which means you can't recruit Cass and complete the quests that involve her. There is no way to fix it. It occurs if you've finished "there stands the grass" before delivering the invoice. He has no more dialogue options. Unless you roll back to a save from before that quest, you will have to start a new game.

How can a game be released with such a MASSIVE BUG??? Aren't there people who are supposed to test the game, don't they release updates and patches or something, this Fallout New Vegas is definitely the number one in the world... in bugs, glitches. slowdowns, crashes, etc...

the freezing and crashes are because of old software that's been defective for a while now. a patch can never fix that, and it's being discontinued in future titles, but quest bugs like that can be fixed in the future. Tluav Boy left a good description of what happened so the patching team may see it and try and keep it from happening to future players(although they most likely are already aware). unfortunately, it is likely that you will have to start a new game to fix it, although it's not a for sure thing.