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From OpenCV Docs :

opencv_createsamples is used to prepare a training dataset of positive and test samples. opencv_createsamples produces dataset of positive samples in a format that is supported by both opencv_haartraining and opencv_traincascade applications. The output is a file with *.vec extension, it is a binary format which contains images.

The .vec is a binary image file. I think it's a vector format of some description but I can't find a source for that.