This is something that I can't completely catch: it says that in 8 days the chances to find it are 50%, and I suppose the number increases while time passes.
So how is it possible that there's no "lost work" if I cut the search now and resume later, when the resuming is actually not resuming anything, but starting from scratch?
Imagine you have a dice with 10000 sides. You want to roll number 4567. You throw it 200 times, but didn't hit it.
You stop for a while, to do something else with your time. Later, you continue throwing 200 more times. You repeat this every day, until you hit 4567.
Does this make sense?
Vanitygen doesn't have a fixed starting point, it searches random addresses. There is no need to keep track of the addresses that were checked already, as the number of available addresses is big enough to never check the same address twice.
Every second that you run it, it increases the chance of finding the prefix you want. If "it says that in 8 days the chances to find it are 50%" when you start vanitygen, that exact same statement is also valid after 4 days, after 12 days, or at any other moment in time. It doesn't matter how many keys you've searched already, and it doesn't matter how long it has been running already.