Never mind: I realized that the solution was to add "vegetarian" to the stop words. I'll leave this post in case anyone else has a similar problem, or feel free to delete it if it's of no value. Thanks again for an awesome plugin!
I'm getting some erroneous results, I think because Relevanssi is not searching the excerpt. (I have WP custom excerpts enabled but not Relevanssi custom snippets.)
If you search my site for picnic, the top result is a page called picnic recipes, and you can see the word "vegetarian" in the excerpt.
But if you search for vegetarian picnic, that page isn't included in the results.
ETA: I guess it's not the excerpt, because I discovered that if I reverse the search terms to picnic vegetarian, I get a lot more results, including the picnic recipes which come up in 3rd place.