Although maybe not as subtle as blue shoes under your dress, earrings can be an interesting way to encorporate your "something blue". Added bonus: all of today's picks are from Etsy!
Splendor earrings by Shopsomethingblue. I love the gold wire contrasting with the duck blue.
Drucilla by lizix26. I have to admit, this blue on this is quite bold but I think with the right vintage dress it could work perfectly.
Little Gems by jetflair. Since studs can be subtle you can push the sparkle factor. You could wear these all the time after the big day as well.
Blue Green Flower Stud earrings by Peaches4me. An alternative to the simple stud above, I love how delicate these look, such detail!
Vintage glam earrings in blue-green by Kristin Friesen. Not quite blue, not quite green, I was totally drawn to the gorgeous color of these. How cool are they?