it seems like lots of vintage bikes lack holes to mount rear racks.

Ever thought of drilling a small hole in the rear frame?

Can't say that I've ever considered that.

What I have considered is getting a frame shop to braze on the attachment points. Or just using P clamps.

P-clips.

Bikes that don't have braze ons for a rear rack were designed for other purposes, mainly racing. Don't drill a hole in the frame. Use clamps or get a bike that was designed to have a rack.

Seems like to me that you'd be, a) weakening that part of the bike, b) introducing a new area for rust, c) most likely not have the hole in the right place anyway.
While the P clamp solution may work, this sounds like the perfect excuse to get another bike.
Don

I'd use old innertube underneath the p-clamps to protect the paint if you ever want to remove the rack.
+1 on an excuse to buy another bike

Ditto - Works well. The p-clips are available at your LBS - 4 Clips will do it. Plus you will need 4-bolts and small nut/washer for each.

+1 on that!
I just cringe when people talk about drilling holes in their frame - especially folks who have a vintage frame and want to install newer water bottle cages. The older, clamp style cages are hard to come by, so people think they can drill some holes and run some sheet metal screws in there.

Really? Perhaps I'm misunderstanding this as the cheap bottle cages I see at the lbs and at wallyworld all have enough room where you can slip a hose clamp up top and down low over where the screw tab is. Again you will want a bit of old innertube over the frame and under the clamp to protect the paint. Perhaps a bit on the jankety side, but completely functional.

Drilling holes in frames seems bad idea to most folks out here.
I considered doing internal cable for rear brake. https://www.bikeforums.net/classic-vintage/561813-diy-internal-cable-routing.html

Adding a piece of innertube would definitely help prevent chipped paint if using clamps. What I meant is that people see bottle cages with two mounting holes, and figure they have to drill two holes in the frame to mount them.
I once rigged up a dual water bottle holder that mounted under the rear of my seat on my Peugeot. Took an old aluminum fan blade, cut and bent it at the right angle, and attached it to the seat rails so I could reach around behind my tush and grab water. Then I saw a commercially available cage carrier like it. Definitely better than drilling holes in your vintage frame!

Step away from the drill....

depends on the dropout design. There used to be a diamond shaped hole in almost all rear dropouts. I have some nuts that are intended to fit in this hole and allow a rack to be bolted to the frame.

If referring about drilling holes in the seat stays for upper rack mounting holes probably a very poor idea. The classic Pletscher cast aluminum rack which clamps to the seat stays is still available as are imitations from Inline and Pyramid. Still would require use of P clamps or the nuts mentioned by unterhausen for the legs if no eyelets on the dropouts.

Bikemania lists the genuine Pletscher version. Amazon has the imitations.

A classic rack for a classic bike.

Another possibility is the Tubus Fly rack which uses a single top strut that bolts to the brake bridge. A lot more expensive however and the top area is very narrow. A minimalist rack.

the guy at my LBS laughed at me when I asked him for a couple of P-Clamps. He knew what I meant after a minimal of explaining but he definitely got a chuckle out of it for some reason.

There are some decidedly non-racy bikes out there that lack the rack brazies.. 4 each Pclamps, 5mm bolts, 5mm nuts should do it. Mebbe some lock washers too. Its not pretty but it works.

I have drilled many holes in steel frames, never observed any ill effects. Holes for bottle cages in down tubes and seat tubes, holes for fender mounting, holes for wiring, and routed a rear centerpull brake cable through the seat tube. I never used sheet metal screws, always a machine thread tap when threads were needed, most often #10-32 or M5. These have always held okay, though the metal is relatively thin. I would avoid drilling a fork blade or stem, but doubt there is any serious structural risk in drilling main frame tubes as long as the holes are relatively small. IMO the best argument against drilling would be the effect on perceived value if you think you might ever want to resell the bike.

Regarding the classic Pletscher rack, I shortened the stays on mine and bolted it directly to the brake bridge rather than the seat stays.

If you're really talking about drilling into a chunky stamped dropout (which would be a logical place if you're mounting a rear rack), I would generally not see it as a problem.