Submitted by PwnedNoMore, also found by Treasure-Seeker
The end function in the Buyout contract uses IERC1155(token).balanceOf(address(this), id) to determine the amount of deposited fraction tokens without distinguishing whether those fraction tokens are depositied by the sellFractions function or by direct transferring. Note that only the sellFractions function is constrained by PROPOSAL_PERIOD.
This vulnerability lets a 51-holder gain the whole batch of NFTs without paying for the rest 49% fractions.
Assume a vault X creates 100 fraction tokens and the market-decided price of a fraction token is 1 ether (i.e., the ideal value of the locked NFTs in vault X is 100 ether). Lets also assume that Alice holds 51 tokens (maybe by paying 51 ether on opensea).
Followings are two scenarios, where the benign one follows the normal workflow and the malicious one exploits the vulnerability.
In short, a malicious users can buy any NFT by just paying half of the NFTs market price.
For each buyout, add a new field to record the amount of fraction tokens deposited by sellFractions. And in the end function, use the newly-added field to determine whether the buyout can be processed or not.
Ferret-san (Fractional) confirmed 
HardlyDifficult (judge) commented:
