Alsa bus in Spain

Maybe?
I've used ALSA busses and I have also used trains in Spain. Which I would use depends on cost and timetable.

The Alsa worked pretty well for us.

Thank you -normally we like the train but these cities have better timetables

Haven't used it for any of these routes, but I was happy with my Alsa bus experiences.
One way rental car would be a pain with extra drop fees. Some people do drive to the nearest town to border, drop car, take bus across to new country, pick up new car and drop, but if bus works; why not?

I had no issue with buses in that area, and ALSA is generally fine. Vigo is one of Spain's dullest cities; minimize your time there.

Zaragoza can be shockingly hot. For many months of the year, I'd consider air conditioned lodgings essential.

Agreed that the bus is more direct and convenient in this part of northern Spain. I've never had nor heard about any particular problem with Alsa.

Alsa is one of Spain's main bus companies; others are often regional, but Alsa covers the whole country. I've taken them various times, and they've been fine.

My Alsa bus from San Sebastian to Vitoria was quite spiffy. I had an airline-style AVOD (audio video on demand) unit at my seat, with movies, music, USB port, and electrical outlet, as well as on-board Wi-Fi! I hope at least one of your buses is like that one (no guarantees).

I took Alsa buses without any problems. THe only thing you need to be aware of: you may need to buy tickets in advance, depending on the travel season and route. I bought mine a day earlier and there's no problem. But I traveled in low season.

Also, check the bus station locations. They may or may not be in central downtown.

The trains being so slow, the bus makes the most sense. I wouldn't stay in Vigo though. Nearby Pontevedra and Baiona are better choices; of course there's Santiago a bit further north.
Between the Vigo area and Oviedo, I would stop by Leon if time allows (in which case there are trains between Vigo and Leon, 6 hours like the bus).

Yes, Leon is a very good stop. It has a large historic area now full of restaurants and bars. I assume the patrons are primarily either locals or pilgrims. The cathedral is known for its glass.

And I liked Pontevedra. Missed Baiona.

Another thing about the buses: There appears to be a policy of stopping ticket sales 5 minutes before the scheduled departure time. They mean it. Larger stations have ticket machines for ALSA and maybe other large companies serving the local market. They're easy to use, and I find the print-the-schedule option on the ALSA machines very comforting when I travel somewhere just for the day. But I once arrived at a station, planning to buy a ticket from the machine 10 minutes before departure, and found all the machines down; I assume there was a network problem. It wasn't a pretty picture.