If I understand you correctly, you want the Boot Kit (set of parts that attaches to the mower deck and on out to the input to the bagger) but not the bagger assembly itself.
If you go onto the Kubota web site and use the Build my Kubota, you can select the
BX25 and in the options select the 60" deck, then the grass catcher. Once you have those, do the Quote. In the quote you will see the tractor, grass catcher, and also the "Boot Kit". I think that is what you are wanting - just the Boot Kit.
I used the Parts portion of the Kubota site next. In "Models" I input the words Boot Kit. A set of Boot Kits comes up, and I selected Open on the one for the RCK60B23BX deck (which you'd want to confirm is the model deck that you have.)
On the left, click on the plus sign to open the parts diagram. You will then see the diagram, with the parts list for the Boot Kit below it. The entire assembly part number is first, K6056-30010. However, you also see that has been superseded with the new assembly number of K6056-30012.
Assembly K6056-30012 will have the deck plate adapter, boot that connects at the deck discharge, tube that goes back to the catcher, etc. You'd have to replace the tube for one that reached your leaf vac, or extend it or something...
http://kpad2.kubota.co.jp/kpad2/PartsInfo.do?bookCd=100A0338&funcCd=&figNo=&himban=&dispType=0
Please don't order K6056-30012 without verifying everything I found...