You're faced with vague feedback on your code changes. How do you decipher and improve upon it?

At first, one should decrypt the vague feedback by communicating clearly and concisely. Once done, convert it into a list of steps and focus on one at a time. Try your best approach to solve all the points mentioned and the unclear issues.

Without a doubt, the simplest and best way to solve the issue of receiving "vague" feedback is to ask for clarification. We don't need to spend an unnecessary amount of time "deciphering" feedback, instead we need to promote clear communication and simply ask our peers what they mean, and if they can explain what their feedback means.

One thing to consider here is to choose your battles, and to "pay back" your colleague for taking time to review your code. There are times when the feedback is reasonable but not high priority to change. E.g., you used 3 lines of code exactly 2 times and the reviewer asks you to pull it into a helper function. This is a gray area for refactoring, the code is fine either way. Hence you could "won't fix" the comment. Consider, though, that your colleague did a thorough review of your code, so you can spend a few minutes and compromise by implementing their suggestion. (Obviously be careful to re-test after making any changes)

ALWAYS ALWAYS ALWAYS re-test after making that last small change...I can't tell you how many significant product regressions that have snuck in due to this.

Reach out to the reviewer and ask specific questions to understand their concerns. Break down the feedback into actionable tasks and address each one of them in your code. Further, request follow ups from your senior and make revisions till you reach the end goal. These reviews provide an opportunity to clarify any remaining doubts and ensure that your revisions meet the desired expectations. This iterative process not only enhances your skills but also fosters a collaborative environment.