I own a Vita and would love to play LR on-the-go, but I'll mention a few key points.
- DoA 5+ while it was a good port for a handheld fighter it got left behind without an Ultimate update never-mind a Last Round update since handhelds are not primary fighting game platforms. Think only the Blazblue games consistently got handheld support, but half the reason is likely because people bought it for the pseudo VN experience. Fighting games on handhelds in general do worse in terms of return revenue. That doesn't mean they can't be viable since I do see some handheld fighters almost sell just as well as their console counter-parts.
- Vita cartridges have like 4GB space max. + was at around 2.5 give/take, but a bunch more content [two expansions worth] has been added since then [several more characters, ton of music, more stages, etc ] which is likely to push the capacity. They could get around that by putting stuff out as DLC like what BBCPE did, but TN might see that as a hassle and didn't bother. I mean TN has fucked up the PC port I doubt they will bother exploring the possibility of expanding their vita version anytime soon.
- DOAX games were previously available only on Xbox platforms outside of a mediocre PSP port. JPN didn't buy them because it was on a platform they wouldn't buy and the Xtreme games got shat on in the West which scared away any chance of mainstream appeal. At least the main DoA games at the time had the fighting game element + males which automatically makes it less niche compared to the spin-off in the West.
- DoAX3 on Vita makes more sense now since it's not a fighting game, the Vita is literally the otaku handheld, and it's something fresh for the platform rather then just an enhanced port of vanilla DoA 5 with some exclusive [back then] features.
- There are a ton of non game related themes on the Vita. Having LR themes doesn't guarantee a LR port.