Hi Emmanuel, welcome to the forums!
There are several ways you can do this which you've already noted here, and let me elaborate on both options:
1. Use existing backup jobs to tape-out the servers
You can simply add the existing primary VM backup job to a tape job and it will tape-out all of the machines added to the job. I'm guessing you will remove these VMs from the primary backup job once the tape-out completes, and the tape job will respond just fine with this. This should actually work with both Simple and GFS Media Pools.
Question: will you be decommissioning these servers after you successfully tape out? If so, I recommend on the primary backup job enable
Retention for Deleted Items. This is mostly relevant if you use GFS Media Pools as GFS will be looking for newer backups as it runs.
2. Create a one-time VM backup job as you described; if this is really meant to be a one-time backup + tape-out of the servers, I would not even worry about the retention [see note 0 however]: just set up the job, let it run once to produce active fulls. Assuming the primary job ran without issue, you can simply create a Backup to Tape Job,
add the one-time VM backup job as a source for the tape job, and let the tape job run. Once done, you'll have a tape copy of these VMs.
3. (optional and not recommended) You can always use a
File to Tape job and just add an existing full backup (.VBK) for each machine you want to tape out. This is probably easiest and fastest to set up, and there's no real problem with it, just the
recovery from tape is a little different. File to tape option is not inferior, but restoring from Backup to Tape job is a bit nicer experience in my opinion as there are a few additional features when using Backup to Tape.
I've written a bit here, and truthfully you had everything correct in your initial post

Let us know if this helps or if further clarification is needed.
0 - Normally there is a
minimum restore point requirement for Backup to Tape; however if it's truly a "one-off" run, the first run should grab all the VBK files as expected. Subsequent runs of the tape job would require the minimum restore points on disk