Some Vignas of interest:
Vigna lanceolata
Vigna vexillata
Vigna umbellata
Vigna trilobata
Vigna aconitifolia
Vigna angularis
Vigna unguiculata
Vigna radiata
Vigna mungo
Vigna subterranea
https://www.researchgate.net/publication/307632448_Interspecifi_C_hybridization_of_Vigna_radiata_13_wild_Vigna_species_for_developing_MYMV_donar
Vigna umbellata seems to cross well with other Asian Vigna species.
V. radiata and Vigna vexillata have been crossed and there's been segregating F2 offspring.
Based off this data, it's possible that some Asian species can cross with African ones. Sweet.
"Vigna mungo is easily crossable with Vigna umbellata." is also mentioned on there.
There's also some mention that some species could be listed into genepools or something.
Some crosses did have pod set, but their pods shriveled up after pollination.
When I grew my potentially hybrid Succotash x Alma's PA Dutch, I actually had a lot of pods abort on some plants.
It's highly possible that some plants will self correct or just up and form pods, with fertile seed in low amounts while other pods die off.
I wasn't aware that Vigna species were known for crossing into each other so easily.
Granted, I haven't read anything on Cowpea interspecifics and that's probably what a lot of people would be looking for.
Some Vignas like Adzuki beans, are thought to have been domesticated in Japan.
Japan has a lot of climates. The beans are even found on some islands with no wild populations or where there aren't many wild types.
I think they've been thought to have been domesticated thousands of years ago before spreading elsewhere.
Japan has similar climates to a lot of the United States. I've found that most natives there do well enough to immediately become weedy here.
So, I'd be interested in somehow crossing some Adzuki beans over to cowpeas at some point.
There is the eventual goal of obtaining Vigna x Phaseolus hybrids.
That's way far down the road.
If I'm reading things correctly, some Vigna species are somewhat promiscuous.
I'll try finding some places that offer harder to find species.