This ends up being more difficult than you might imagine. The switch from 27 inch to 700c is pretty easy with calipers but you may not be able to move the brake pads 4 mm with a lot of cantilevers. An additional problem is that the brake pads on a vintage bike tend to be closer together than on a modern bike which means that some modern cantilevers won't work right on the bike.

That said, this is doable but you may have to try different cantilevers before you find one that works. I did this once on a 1983 Trek 720 which was designed around 27 inch wheels. I ended up using modolo cantilevers which worked with 700c wheels. Those are tough to find. I've read that others have had good luck making this conversion with the Deore cantilevers.

Here is a useful thread on the topic,

canti brakes for 27" to 700c conversion?