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 !
After some .png and .yaml renaming and .yaml editing, the merging was successful.
Thanks for the tips.