1. WoD games are set up so that they punish you for rolling dice; your RP 'should be good enough that you don't need to roll dice', you simply narrate what best fits the scene, and the Storyteller adjusts it as appropriate.
If you want to roll dice, any given die has a 30% chance of success (70% chance of failure). Chance dice have a 90% chance of failure.
For a Simple Success, the chances of failure are 70%, 49%, 34%, 24%, 17%, 12%, 8%, etc (7^x / 10^x where x is the number of dice in the final die pool). This is for final dice in the pool, not for the initial die pool (penalties subtract from dice rolled, before the roll). For success, simply subtract those from 100% (i.e. 30%, 51%, 66%, 76%, etc).
For an Exceptional Success, the probabilities are a lot more complicated due to X Again rules. Whether the roll is 8, 9 or 10 again has a serious impact on the probabilities of exceptional success.
2. WoD is all about the story. Everything is subservient to telling a good story, especially dice rolls. That said, don't rail road your players, and don't go around inventing mechanics to disguise your Storyteller Fiat; I've played in campaigns that made those mistakes and they are still horror / joke stories I share with other friends.
If you do choose to let dice get rolled, my advice is to follow what the dice indicate. In my experience, dice rolls tend to be better narrators than any gamer I've played with. The sudden shifts in fortune, new injuries, success and failure. The only exceptions I've come across have all been in WoD games.
Good Luck.