The security I have read about is there is a unit which goes out to purchase food for the President and supplies the White House kitchens, and it is clandestine. No one knows who they are or where they go to make the purchases. It’s always different. So the team may have had a security breach.
The White House kitchen prepares about 50 meals a day on a very slow day. It is impossible to know what food is going to end up where. So let us say you manage to sneak in a poison bottle of vanilla. The chances of it making the president alone sick is beyond the realm of possibility unless he is given to chugging the stuff.
If you wanted to actually poison the president you would need someone in the kitchen or a server to poison his plate and do it under the eye of the Secret Service.
I am not saying it could not happen but it would be very hard.