Prefixing any search qualifier with a – excludes all results that are matched by that qualifier.
For example, you might be interested in finding all “cats” repositories with more than 10 stars that are not written in JavaScript:
cats stars:>10 -language:javascript
You might also want to find all issues mentioning @defunkt that are not in repositories in the GitHub organization:
mentions:defunkt -user:github
The answer for your question is:
is:pr author:limonte -user:vaadin
For more refer the GitHub Search Syntax