By finding collectibles, killing enemies, completing objectives you earn XP and with every 1000 XP you gain a Power point. When you press you can distribute these points among your vampiric powers. In order to learn and upgrade all powers within one game you either have to kill a lot of enemies silently and find a lot of collectibles or you can choose the easy way and exploit a section in chapter 3 to get unlimited XP and a fully upgraded character.
You will be in a huge area with a round bar in the middle. If you run straight to the back of this room, behind the stairs you will find a PDA collectible. Pick it up and then run to the two guards chatting nearby so that they notice you. Though you will find yourself back where you started, the 200 XP for the collectible will not be lost and you can head to the same PDA to pick it up again for yet another 200 XP. Like this you can gain a level every 2:30 minutes.
You need altogether 64,000 XP to buy everything and fully upgrade your character. To minimize the grind, you can stop between 40,000 - 42,000 XP as after this section - even if you do not focus on stealth kills - there is enough XP in the game to take care of the rest.
Remember, if the exploit will be fixed in the future, you can still delete all game updates from your hard drive, go offline, use the exploit and once you have upgraded your character you can download the game updates again.