Restarting Vanitygen
Vanitygen works like this:


Generate a random 256-bit integer to use as an ECDSA private key
Compute the corresponding public key
Compute the hash of the public key to find the address
Check if it matches the requested pattern (1Mathias in your case)
If yes, halt and output the private key; if no, start over at step 1


Notice that there is no state saved in this process, so you lose nothing in stopping and starting as much as you want.  Each random private key has an equal probability of success; it doesn't matter how many you have already tried.

For a given number N of random private keys tried, one can compute the probability that (at least) one of them would succeed.  This is the "percent" number that Vanitygen reports - it's not a percent completion!  In the excerpt you show, it says that it has tried 2888131518 keys, and that mathematically, trying 2888131518 keys has a 10.2% chance of finding one that works.  It also says that in 18.5 hours, it will have tried enough keys that the chance of success is 50%.  This does not scale linearly!  If you are unlucky and don't find a key for 37 hours, it won't say 100%; it will say 75%.

As an analogy, suppose you have an ordinary 6-sided die, and you want to roll it repeatedly until you get a 6.  Your chances of succeeding with one roll are 1/6 (16.6%); your chances of succeeding within 10 rolls are 83%; your chances of succeeding within 100 rolls are 99.9999987%.  But this number never gets to 100%; there is no number of rolls that will guarantee success.  There is always some chance that, of all the rolls you made, none of them came up 6.

So the "percent" number reported by Vanitygen is never going to reach (much less exceed) 100%.  It is simply intended to give you an estimate of how long the process might take, but since it is random, there can be no guarantees.  If it gets very close to 100%, all you know is that you have been very unlucky; but your odds of success with each new key are exactly the same as they always were.