Can I set the copy backup job restore points to keep differently to keep full VBK static until the actual backup job does another full backup (maybe using GFS option??)
No, it's not possible. Once the specified retention policy is exceeded, the full backup starts moving forward on regular basis.
How does the backup to tape functionality work? I have not been able to play with this yet as library still assigned for SBE. Does it look into the VBK, VIB files and can it do an incremental compared to what it transferred before?
Yes, backup to tape job works exactly how you've described, copying only the restore points that have not been copied to medias previously. However, please be aware, that with current implementation it's not possible to use backup copy job as a source for backup to tape job.
Any other ideas?
How often do you need to archive backup files to tapes? If it's possible to copy backup files on less frequent basis (say, weekly or so), the combination of backup copy job and file to tape one might be a valid option.