@aditbeat
Hello,
The field/uploader for adding the mp3 files would have to be coded into the front-end product add/edit form for WC Vendors Pro for that to work.
If it works fine with WC Vendors free for the vendors to use, than you will need to find the meta keys and meta values that the mp3 plugin adds to that field, and then build in a form for uploading those in your functions.php and also in the product add/edit template for Pro. We have hooks throughout the Pro template product-edit.php that can be used for placing the new fields into the form.
This may be helpful as you look to add to your product add/edit form in Pro:
http://docs.wcvendors.com/knowledge-base/adding-a-custom-field-for-products/
And this example shows how a user worked in a similar plugin to the front end forms (his code is not formatted properly, but you can see how he did it):