I suppose that if I were to define the difference it would be this: Transvestic fetishism is when a guy puts on women's clothing to get excited, gets his jollies (with or without a partner) and then removes said articles because he's finished with them.
Being transgendered, and I'm using my younger self as a model, is that after the fun, the clothes stay on because you feel more comfortable in them than out of them. It's more than just the sex even if you don't want to transition and you still identify as mostly male.
helen