If it shows like that its indeed most likely here are again some works along that line-could be in PT or in ES. The spanish have a fairly goof site showing it-adif.es.  If it is the case, then the replacement bus is-as always-usable with the pass. Also check-just to be sure, what cp.pt says.

In ES it is very hard to say in advance-and from a very recent report from someone who was charged extra to use FEVE trains-felix becker- he also told it was not possible to do REServ on RENFE in PT. The usual way that spaniards handle is that to call RENFE-state wishes, get a client/order nr and you have to visit a counter IN 24 hrs after!! to get tix printed and paid.

Mad-BCN has hourly AVE trains (usable with pass), and also a few NON-pass but often quite cheap low-budget hi-speed trains by AVLO and OUI-GO-it may work out cheaper as using a passday. Once in ES-and having the odd more as 1 min spare (unbelievable how long any booking takes on RENFE) you can get a peaceful mind in advance.

There is always stil the 1/day and often very late local regional express Mad-BCN via the old line-taking some 12 hrs, but its an old-style NON-AC set and it can get desert-like hot often in MAD and middle Spain.

The best is to book as soon as you arrive in Spain. This can be done at many stations, it doesn't need to be at the station where you board the train.

Thanks to all who have replied, it is all helpful information!

Still wondering: are these trains likely to be fully booked in July?  We won’t be in Spain until less than 24 hours before we want to travel from Vigo to Barcelona, so we can reserve a little bit in advance, but not much.  

I can't comment about how full these trains will be - in addition that often also depends on the day of the week (Fridays and Sundays are often busier).

You could try to book a bit in advance by making a prebooking by phone via RENFE (scroll down to Spain). You will the have to pick up the reservation within 72 hours after prebooking but that's better than waiting until you get to Spain.