As someone who has flown both VARA A320s, I don't think they are a step up from anything. The interiors are shocking. One is much worse than the other. No entertainment, windows cracked and opaque, worn out and broken seats, etc. Virgin should be embarrassed to have their name on it. Might be the worst commercial plane in Australia.
Apparently the eldest one (VH-FNP, 1993 build) is subsidised by Airbus as a case study for airframe life!
It appears to be one of the few (still flying) A320's of that vintage not operated by a big airline such as LH, DL, UA or AC.
There are so many A32xNeos and 737-Max being built right now so I imagine lease rates for late-model A320/737NG must not be too bad - hence no rush to migrate Tiger.
My armchair CEO guess is that the whole Tiger 737 thing was a play to get out of any unfavourable/expensive A320 leases done by the previous owners.