Unlike in the Sims 2, each save file exists in its own, separate universe.
In the Sims 2, time did not pass for sims you weren't playing. However, in the Sims 3, the "Story Progression" means that no matter which household you play, time will pass for all families in the neighborhood.
The one thing that is shared between neighborhoods is the Family bins -- that is to say, both you and your fiancee should have each other's sim in their neighborhood bin. If you then move this sim into the neighborhood (even if you don't change the active household), you will be able to find, meet, seduce, and eventually marry each other's sim.
However, because you have separate save files, marrying in one neighborhood won't simultaneously marry the sims in the second neighborhood.
It is possible to simply play multiple active families in the same neighborhood, but since every household ages at the same rate in The Sims 3, this will probably not be a preferable way to play (not to mention losing wants every time you switch active households).
Yes, it is possible to make other Sims break up. If you perform enough romantic actions with the Sim you are interested in, you will have the option of asking that Sim to break up with his or her partner, assuming the Sim in question is now a "Romantic Interest", as opposed to "Friend", "Good Friend" or "Best Friend". You will then be able to ask them to go steady, propose to them and eventually marry them.
Keep in mind that by asking a Sim to break up with their partner, you automatically make yourself an enemy of that now ex-partner.
Best Answer
Simply put; No.
For occult status, only the parents are checked.
Of course, if the human child were to later become a vampire, their children would be able to inherit it. ;)