The sub is probably abandoned by this point but wanted to ask if anyone heard back from CUNY, Mannheim or Cologne?

You do not need to publish papers for a PhD application, at least not if you are applying to schools in the United States.

Hello everyone, I am form Vietnam and I am currently planning to pursue a PhD in Economics. I am looking for peers or professors/mentors with more experience to collaborate with and share experiences in the field of Economics. I also have some ideas related to Agriculture (rice cultivation) using VHLSS data (household survey) Since I need to publish some papers for my PhD application, and I have some research experience, I hope to find a partner or mentor. Thank you very much!

Here is a little sad data that will not surprise anyone. Admissions to econ PhD programs were down 12 percent from last year.https://pubs.aeaweb.org/doi/pdfplus/10.1257/pandp.116.720

Students at American schools are more likely to do a pre-doc and not a master's, while students elsewhere are more likely to do a master's and not a pre-doc. You might want to take a look at placements from your current program to help decide whether a pre-doc is really worthwhile for you.