Firstly, Vivi is a mana-making machine, able to spawn additional mana based on its Power value
Question How do you know what color is right for you
Specifically, would you risk losing if it meant you could defeat your opponents
Throes of Chaos would always cascade into Tibalts Trickery, which would counter Throes and cast an Emrakul, giving you an extra turn to annihilate your opponents board
However, you will need to pay at your next upkeep, or you'll lose the game
While I try to avoid giving ridiculously low offers in an actual negotiation (just putting an offer out there knowing that you'll walk if it's rejected is not negotiating), it's always best to aim a little bit under the price you are will to pay to give yourself some room to up the offer