1. I am trying to make a Venture, what should I know about the class in the case of Advantages and Merits/Flaws/Freebies?
2. How does the Merits/Flaws/Freebies system work exactly? I haven't seen a list of Flaws I could choose from and how many points they add to the total 15.
3. Do I make up the Merits/Flaws?
4. I want my character to be REALLY proficient in Fencing. Can I do that?
You almost certainly want your Ventrue to have a huge number of Backgrounds. Allies, Contacts, Resources, things like that. You get 5 background points just for showing up but can buy more with your freebies.
Merits and Flaws themselves start on page 479 of the 20A book.
As far as really proficient, you take 4 points in Melee (buy 3 with your starting points and use 2 freebie points to buy a 4th) and you get a Specialty. Have that Specialty be fencing.