I understand you are supposed to replace the cartridge once a year- but when I search for it I can only find the whole ozone generator assembly (which I don't think needs to be replaced once a year....) am I searching for the wrong part?
A quick search online of the Vita spa line of hot tubs shows that they use the CleanZone ozone cartridge generator (plasma gap corona discharge) made by MAAX Spas. However, it doesn't appear that the units are available for online purchase and that you need to go through your hot tub distributor to get a replacement unit.
An annual replacement schedule on an ozone generator could be an expensive proposition. Unfortunately there is no easy way to know if a unit is generating ozone so you're stuck with the expensive/frequent replacement cycle particularly if you are using the ozone generator as the means for regenerating your bromine sanitizer levels from a bromide bank in the water.
You might be interested in our article on how to convert a hot tub to chlorine which is A LOT cheaper to run than a bromine spa - How do I use Chlorine in my Spa (or pool)?
Many TFP hot tub owners prefer the dichlor-then-bleach method as it is a lot easier to implement and test for and uses the same chemicals and test kit as the pool does.
Bubbles are not an indication the unit is producing ozone. It's an air flow-thru unit so it will suck in air and make bubbles anytime.
Also, ozone by itself is not an appropriate spa sanitizer. It's pathogen kill times are too slow and lack of residual concentration in the water make it ineffective. You are st risk of developing pseudomonas (i.e., hot tub rash) or some other nasty pathogens. If you want to use a halogen free spa product you can try the Nature2 mineral system which uses silver ions in conjunction with MPS. It's the only non-halogen based sanitizer aside from baquacil that the EPA allows.