After some .png and .yaml renaming and .yaml editing, the merging was successful.
Thanks for the tips.
So I've asked around and this isn't something that has been done before in SBRE and the experts/creators of this format have now left the company. However, someone over at SBRE was kind enough to give some insights on how it could be done, by having a look at the code of the .vrd file. Be aware that the method indicated isn't guaranteed to work ! It is merely a theory formulated after the code was looked at.
Here's a retranscription of the info I could gather :
For each objects database, this format is made of a file with all the required information :
This file is compressed into a tar.gz format by Choregraphe, the resulting file gets the extension .vrd, therefore a .vrd is a .tar.gz.
Schematically, in order to merge 2 .vrd files, you'd need to :
Hope this could give you a few leads on how to solve your issue !