Well, RedKilley, let me address your second point first.
2. Giving back the Blinking Mirror to the FC (which was your only evidence) didn't really prove that she was born after the event in Suikoden 3. It might show that they met after S3, but it's possible as well that she blinked from some era into the time after S3, and then meet the FC after that instead.
This is true. I guess I made an error when I said 'born after Suikoden III'. I DID actually mean to say that they MET after Suikoden III. Because of her blinking magic, you're right, we have no idea when she was actually born.
As for the stars, my personal belief was that having the two Viki's under different stars was an error on Konami's part. For the sake of argument, though, let's say that they did that on purpose.
In which case, my argument IS void. I guess it is possible that blinking around so much has not only messed up Viki, but also messed up her fate, thus changing her star. There are a lot of mysteries in the Suikoden world that we know nothing about and that seem to break the rules that Suikoden itself laid down. (i.e. Jeane) However, as much as I would like to think that, it does seem like a bit of a stretch to me.
So, in closing, I guess my theory is based on what the two Viki's having different stars actually means. Obviously, I believe my theory, so I'll be searching for ways to back it up come Suikoden IV. Until Konami makes up their minds to tell us, however, I guess we really won't know.