@Dan2 you can link legacy VPP acocunts to ABM
We had the same thing seperate DEP and VPP account before Apple released ABM and ASM, we linked both accoutns and now all is well. There should be a support number you can call if you get stuck.
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.