I think Vermouth just taking some surgery for her appeal
If she swallowed an APTX 4869
Then whole BO will realize that APTX is not a killing machine
but for a Shrinking Machine
and soon if Vermouth get shrink by APTX 4869 then Gin should realized that Shinichi is still alive and live as a Kids somewhere .
Or maybe Vermouth Swallowed an APTX 4869 as a testing materials for the First time APTX 4869 was found
and She shrunk, finally The researcher maybe both Sherry(Haibara) and Akemi (Haibara's Older Sister) fixed the APTX 4869 and said that they add something on it (IN fact is not)
Then after finishing the task Akemi got killed in the Robbery case, and Haibara decided to swallowed APTX too
but I don't think that vermouth Swallowed APTX 4869
If she swallowed APTX , she would realized sherry shrunk and recognized a shrinken sherry in the Bus Hijacking Case