Well, I think the PM should be able to shed some light on the vacancy mystery. Don't give up until you get a clear answer that you are happy with! There's always a reason.
I've never done a seller financed deal, nor have I ever heard of anyone getting a 0% seller financed loan. Not to say it's never happened, but from what I know, if you can get 0% interest, you should consider yourself very, very lucky. With that kind of offer you are giving them very little incentive to finance the property, unless they are really desperate to get out of it (or there is something seriously wrong with it).
Making the seller an offer is really almost an art, especially if you are trying to be creative. You can lowball them, just make sure that whatever price you come up with, there are analysis numbers to back it up. You want the seller to know that the price you came up with is a result of analysis and not just handwaving - they'll be a lot more likely to negotiate with you in that case because you will have specific income/expense items to discuss. Otherwise, you just risk pissing them off :-)