As it goes for most countries, it's good to know a few useful phrases in Korean. But in Seoul, you'll find that at least most signs are in both Korean and English. If you use the subway, the announcements for each stop will be in 4 languages. Most Koreans are shy about speaking in English but they speak English pretty well (IMO). If everything fails, just call me!
While certain dishes by nature are vegetarian (tofu), you'll find that most Korean dishes involve meat. That being said, I have a few restaurant recommendations if you are vegetarian/vegan. It won't be as hard as you think!
https://www.budgetyourtrip.com/south-korea