I doubt you can easily merge these partitions. Even if you could, I am not sure I would risk it on a production server. The issue is that the full partition is your root partition and re-sizing the root partition may have to be done offline and by your host. The method varies depending on the virtualization software in use.
Unless you have a large number of accounts, why not migrate to a new VPS with a larger disk?
If you don't want to migrate, consider moving /home and /var/lib/mysql to your new disk. You can then delete them from main disk, freeing up space.