It's completely safe to delete old VRB files, since in reverse you always have the full one as the newest one.
I see anyway a problem in Veeam deleting old VRB files for two reasons:
- you are violating your own retention policy
- Veeam cannot know in advance how much space a new backup will occupy, so it will have to do it "on the fly" while backup is running. Not impossible, and maybe a good feature for the future. If you know you will violate your retention, you can choose to do so.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1