How do you soften up a new t-shirt to give it a vintage feel quickly?

Have you tried soaking it in tea or coffee?

Soak in vinegar. Then wash a few times and add fabric softener then use wool balls in the dryer.

I would think that the fabric content may be to blame. Cotton gets soft but polyester doesn't.

I use liquid softener when washing cotton t-shirts. I guess I am wondering what you mean by vintage? Old? Faded? Soft? If soft, use liquid softener and be sure to dry your t-shirts in the dryer until they are dry....do not put them in alone....just with other like fabrics/colors.