CDF works the same way in both cases (all CDF always work the same way).  In theory if you have infinitely bad luck you could never not in a billion years find a solution for your vanitygen prefix.   That is very unlikely but it could happen.  For Bitcoin mining the fact that the previous block changes periodically makes no difference.  There is no "progress" towards a probabilistic solution.  You either solve it or you don't.

In both Vanitygen and bitcoin mining if your 50% CDF was 1 hour there is a 50% chance you will find a solution in less than or equal to 1 hour.  This doesn't mean you have a 100% chance in 2 hours. There is no 100% chance even in 1,000,000 hours (although it might be 99.999999999999999999999999999999999999999999999999%).

Also I assume it is just a typo but in vanitygen (just like in Bitcoin mining) you aren't looking for a specific unique solution but rather one of any number of the quadrillions of possible solution which match the prefix.