I think I understand your question, but I may need to make some assumptions and ask some questions as well to help you most effectively.
First, I'll assume your GB project is following the DMAIC flow and since you're asking about correlation between 2 variables, you are most likely nearing the end of Measure or perhaps working on Analyze at this point. Hopefully not still in Define based on what you're asking.
If I'm close, you should have asked and answered a few questions by now - 'answered' via tool use. Specifically, you might have a detailed "as is" process flow map so you can "see" how these vials actually get filled. Given the whole 'Y = f(x)' concept, I assume yield is your primary metric and therefore the "big Y". A very common Measure phase exercise is to assess cause and effect via a cause and effect diagram (aka Ishikawa, or Fishbone chart). By asking your team "Why do we see variable yield?", you should be able to (or already have) populate a C&E diagram with many potential causes (x's).
Since you are already collecting data on several, look to see if any of these show up as likely root causes on your C&E chart. You should be including subject matter experts - people who run this process and have a fair amount of knowledge to complete the C&E.
The point here is that based on the way you've asked your question, we have no way of knowing if you have used any process-based tools to populate the list of probable root causes - hopefully you have and the list of data elements is not just what you could collect easily.
Once you do have belief that you have the most likely subset of X's, you can run simple scatter plots (aka x-y charts) to see graphically if any apparent correlation exists between your X's and the Y. I would also suggest that you get help from a BB or MBB to determine if there is also correlation between the X's as that can pose a problem with your subsequent analysis.
I hope you follow up with this - happy to help you work it.