Hi Dog
Its great that he's happy and sociable, if he's like that at nursery, there's no reason that should change when he starts school. My ds1 has never been happy and sociable at school, so its definitely something to be valued.
We have been through the loop with the vacant thing as well and ended up at the Paediatrician in January after the GP queried possible absences and referred him. The paediatrician felt that the vague episodes were more of a coping strategy when things got too much for him. He got him to do a quick stress test (weirdly, blowing a piece of A4 paper held up in the air continuously for one minute) apparently in most children with absences this will bring on an attack.
I was the same as you. I saw him through the classroom window soon after they started this academic year and it really frightened and upset me. Ds also had a drop attack during a maths lesson and blanked out and fell in the playground, but the paediatrician is still convinced its just stemming from stress or sensory overload. We're keeping a diary of when it happens now so that we can see if there's a pattern or common theme. (I did go on YouTube and watch some videos of absence seizures just to put my mind at rest though.)
I have heard of quite a few children with language delay over on the SN board who have other issues that are basically down to frustration at not being able to communicate clearly and be understood. However, his language skills are improving and will continue to do so. I know its hard, but try to let go of your worries about his future and concentrate on the here and now. Try to hold onto the fact that when he's older he won't have the same issues as he does now - he will move on, develop and improve.
There was a lad in ds2's class that had language delay, very few words and what he could say was very difficult to understand, but it didn't bother the other kids in the slightest, in fact he was very popular because he was kind and fun to be with. My ds2 started school with a huge swollen eye with a massive infected lump on it the size of a marble, I was so worried about how he would be treated, but the other kids didn't even bat an eyelid at him. In my experience children are far more accepting than adults.
The hearing test is a good idea, to be doubly sure there's no probelm there. Have you considered that some of his problems might be caused by sensory overload - bearing in mind the zoning out? I know you mentioned in the past that he had a problem with loud noises and echoey rooms etc and wonder if that might have something to do with him shutting down and being vacant. I wonder if it might be worth asking for an occupational therapy assessment (making sure it includes a sensory profile) to rule this out as well?
Its good that they are already considering planning his transition and making it as comfortable as possible for him. If you're still worried, ask if the SENCO (is that the inclusion officer at your school?) can go in and do an observation of him in his current setting, as in truth, she's probably better suited to noticing anything that needs further assessment or investigation than his teacher.
B sounds like she's doing really well. Its a double edged sword isn't it? We are the same. Every parents' evening we are told what a lovely boy ds2 is, how all the children like him, all the teachers wish then had more boys like him in their class and how he's making good progress. Then we have meetings like ds1's review today (he doesn't have parents' evening as he's on the SEN register) and the gulf between the two of them seems enormous. I think it will be even more obvious with dd when she starts school, as she's bright as a button just like ds1, but with good social skills etc and real desire to interact with everything in her world, which obviously he lacks. The only positive I can think of is that it can't be my parenting - its cold comfort though. 
((hugs))