Optus mentioning it went to Vodaphone makes me think they maybe the number was originally from Vodaphone. So it wasn't that someone else ported it out to Voda but instead it was originally frmo Voda so after the standard 6 months in quarantine, Optus released it back to them which is standard practice. It's a long shot and Voda's processes might not allow for it but maybe try calling Voda and asking them if they could sign you up to a pre-paid plan with your number. If they can, once activated you can port out to whoever you want to use.
The ombudsman seems unlikely to be of much help in this.
You only 'own' a phone number for as long as you pay for it. If you stop paying then the Telcio is free to use it as they want. Most provide a six month change of mind period but after that it can be anyones. AFAIK for any numbers (landline or mobile) you can't request specific numbers from the pool (even if that number is currently available)