This is easy; keep the two vintage bikes and get what you need for commuting. You can set up the 520 to work with 700c wheels. Try out some 700c wheels and see if the cantilevers adjust down. If they do, you're good to go. If not, do a bit of research and find some cantilevers that will work. These are both great bikes; you will not find a better new bike.

By the way, I own and ride a 1984 610; it's a great road bike for all purpose riding. I also own (and I'm in the process of fixing up) my 1983 Trek 720 (the precursor to your 520). I use it as a touring bike but it makes a great commuter as well. And I run 700c tires on it.

If I were looking for a gravel bike, why not a vintage mtb? Those make great all purpose bikes. Or simply use your 520; a touring bike works well as a gravel bike.