I hate Piazza Risorgimento because I love it. Every time I walk by I am crippled with doubt: should I go to Knick Knack Yoda or 200 Gradi? It’s a tough choice and I honestly would recommend accepting your inner hunger and just going to both places.
200 Gradi prepares some of the absolute best sandwiches in Rome. They make their own bread (baking it at 200 degrees, the translation of the name) and have many top notch ingredients from all over Italy. Each sandwich has the name of a square of Rome and there are so many different “panini” they use names of squares I have never even heard of.
In addition, they have a good selection of bottled beer and craft soft drinks: try the Lurisia Lemonade and discover what Sprite should actually taste like. The staff is friendly and behind the corner there’s a good but touristic ice cream shop called Old Bridge. You really have no excuse to not stop by after a visit to the Vatican museum.