I have read several articles saying how you can make your new t-shirt old in a matter of days using a salt water soak. Other articles would have you use additional items like baking soda, vinegar, or if you are in a particular hurry to see results, sandpaper.
I have a couple of questions and opinions on the subject.
Oh, I can get it if your goal is a softer more comfortable shirt and the design you want is not available on a softer shirt. Maybe your skin is delicate and easily irritated? But artificial aging doesn't make a vintage shirt, at least not in my opinion.
What really makes a t-shirt vintage? I think it's the memories of where you got the shirt and what was going on, and especially, who you were with. Also, you may have accumulated many other memories while wearing the shirt over the years.
I have never called any of my old t-shirts “vintage” Very few last that long, but I did keep two that have some memories attached. One is black with a white print of Frank Zappa's head and the words “ My Pumpkin” at the bottom. Very cool. I got it at a Zappa concert in the 70's. The other is a island scene with a sailboat on a purple shirt I got on my honeymoon. Unfortunately, now they are a smidge smallish. Hey, I grew up! But, you see these are a good and a great memory with people I liked and still love. That makes them special. Call them vintage if you like.
Back to the methods suggested for cheating time and wear and tear. It is suggested to soak the shirts for several days in brine water perhaps with a bit of baking soda for best result. You can add some tea if you want to tone down the color or make a white shirt look awful. Then if you are feeling a bit aggressive and impatient, break out the sandpaper or emery board.
As they say, to each his own. As for me, I say, wash it when it gets dirty and call it good. Only when you look back over time will you know if it has become a true vintage t-shirt. If it lasts that long.