Woah. I think I've just entered a parallel universe
This reminds me of Back to the Future, which explains how I managed to follow your plot. Okay, there is one variable you have to think about before you go any further. Is the George from Vista's original world still the practical jokster we all loved from the books, or does he have a completely different personality than we know?
If the George from Vista's original world was different than the one from Harry's world, then it could be completely plausible that he would hide a secret engagement. The George we all know would NOT, however. The Weasley family is very tight, and as much as George and Fred snuck around throughout the books making their joke products, I don't think George would be able to keep something like that a secret. Why would he need to?
Another problem you may run into is the fact that the new George doesn't even remember the Vista from the old world, because everything has changed. He may think that a random girl that claims to be his fiance is really *weird*, or he may be flattered. It is hard to say because I don't know how you want the story to turn out. This is definitely AU, right?
Now if Vista became friends again with George, it is very possible that he may like her again, but it will be a different *like* than the world Vista knew him in. I know this is a very complicated topic, so if this made absolutely no sense to you, please PM me and I will try to explain further