by default openframeworks flips the y coordinate compared the the default opengl where y grows upwards instead of downwards. in some cases, though, like when using a camera there’s no vertical flip and y grows upwards.
Depending on that the bounding box of the string will be different.
So if you have the default perspective in OF then you don’t need to specify vflip. If you are using a camera or somehow setting the perspective manually then you would need to specify this parameter and set it to false