Last year we had a deadbeat tenant who was way behind in rent and then bailed on us in August. The place was dirty and damaged when they left and we haven't been able to rent it since. So this I think is two questions - 1) can I claim expenses (mortgage interest etc) while it was vacant August - Dec? ... And 2) What will happen when I claim essentially zero income as they had effectively paid for Jan-Feb before they bailed.

I imagine it will raise a flag or two.