Fit & Flair
Full head-to-toe vintage reads as a costume. One vintage piece inside a mostly modern outfit reads as taste. That's the whole trick behind mixing eras well.
Pick one anchor. A graphic tee, a jacket, a single loud piece, and build modern, simple basics around it. The anchor gets to be weird because nothing else around it is competing for attention.
Fit is what sells the mix. An oversized vintage tee tucked in and paired with fitted modern pants looks intentional. The same shirt worn baggy top to bottom with baggy everything else looks like laundry day.
Don't be afraid to tailor a thrifted piece into your modern wardrobe. Hemming, taking in a waist, or swapping buttons on an old jacket makes it stop looking like a costume piece and start looking like it's actually yours.
Texture contrast works in your favor. A rough vintage denim jacket over a clean modern knit, a worn band tee under a crisp blazer. The old and new pieces make each other look more deliberate, not less.