Design & Interface > Fashion > T-shirt Graphics is the correct category.
Although the vector would be great in the vector galleries, to give yourself maxmium exposure please post these sort of deviations in the T-shirt graphics gallery.
Due to the nature of t-shirt graphic design, the viewer sees it in a different way. You're more likely to use a more simple design and palette than you would a piece of vector art. If it's in the vector art galleries, it's not viewed in the same context as you would the t-shirt galleries.
If a user is looking at great t-shirt designs, he/she will look in the t-shirt design gallery. Know your audience <3