Tl;dr: All yearn vaults are working as designed, including the TUSD Vault. All deposited funds in the Vault remain safe and are earning positive yield.

On Monday, September 21, the TUSD Vault allocated $10m in funds at once which caused slippage on the transaction and thus getPricePerFullShare became worth slightly less than it was previously. Since APY calculation depends on the difference of per share prices, this made APY calculators incorrectly display the APY as negative. No deposited funds in the vault were lost. Rather, the vault's earned yield was reduced, resulting in a short term dip in the share price. A function that is meant to be called at times when it is determined profitable to do so, was inadvertently called. We are reviewing what happened, and the measures that would be effective in order to prevent this from happening again.

As with any currently active yearn vault, you cannot lose what you put in. The value of your deposit will either stay the same or increase over time, and you will profit as long as the yield you have made by the time of withdrawal exceeds the 0.5% withdrawal fee.

This had the effect of showing both higher than normal apy and negative apy, but this should be normalizing again. Also no the excess 15-ish million is not what caused this, but the 10m allocation. You can read my comments about how the tusd vault holdings here