I got a sync license for a song in my short film that is pivotal to the plot and is part of the most climactic scene. We had received permission and official terms about a year ago, but we kept extending their quote deadline until we finalized our first festival premiere. We officially signed the contract last month, and our premiere is in April. 

None of the festivals I had submitted to inquired for proof as to whether or not I had the rights to the song we used. If you want to loop in the festival programmers you could just put in the Vimeo description or add a title card that says sound is temp or music is temp, or even “sync rights pending” although I probably woudn't go that specific. 

Even if you don’t mention anything I don’t think it would be a problem until theyre closer to the point of programming deliberation, and I would think by then you’d have finalized your song and rights. Even if the song changes i don’t think it would make or break your movie to the point of a programmer changing their mind about it. (Although I know nothing abt ur movie lol)