The synth is free. Paid levels seem to be mostly for skins/presets/support. There are more wavetables in the paid version, though I don't know enough about wavetable synthesis to know how much that matters.
I haven't registered and downloaded the software from their website but I doubt it would come with any licensing or OSS mention. The marketing doesn't mention OSS either.
There's at least one thread asking for clarification on the subject which, as far as I can tell, is not formaly answered.
The repo you linked is 13 months old but it's informative nonetheless:
> Do not distribute the presets that come with the free version of Vital. They're under a separate license that does not allow redistribution.
So my best guess is that the author is granting himself an exemption and distributing that software as a closed source software. Which is legal, it certainly helps that there's only one author.
If we really wanted to know what's going on exactly, we should download the software from their website. If it's advertised as being OSS and under the GPL, we should ask for a copy of the source code of the distributed software, as provided by the GPL.
The software's copyright holder cannot infringe his own copyright by distributing the software from his own website, even if he also offers a GPL license. If he were accepting contributions from other people you might have a case, but he says he isn't.
The GP is saying that author is distributing that software as unfree with a different license since as copyright holder, the author can do whatever they wish. QT, for example, is distributed as GPL or with a commercial license.
Which is to say the GP is correct this is possible and it's a sleazy maneuver.
Which is unfortunate since I actually am playing with Linux audio software and I'd love to find a good free software synthesizer. LMMS is OK but this does seem pretty impressive.
What do you think of Vitalium? I don't know enough about synthesizers to have an informed opinion.
The problem is these people saying they're open source then playing game pisses me off much more than if they said they were commercial with a free version.
So I'm torn on trying them. And I can't get the github code to compile on Ubuntu 20.
LMMS is a digital audio workstation, not a synth. If you're looking for another FOSS DAW there is Ardour.