HI, I suppose to know the answer, but I thought about it…
how to enter “vocals engineered by” into MB?
It’s possible to select a sub-category of vocals. In my case there would be (at least) lead vocals, background vocals and choir vocals. Should I enter all of these? It would be accurate.
For
Who “vocals & Guitar” engineered by? Linking the edit is confusing without those details
I would just select sub-category of “vocals” as they are clearly engineering all of the vocals. Personally I’d see naming each item as clutter.
… engineered by Chris Bittner and Mike Birnbaum (misspelled as Birbaum)
There are details: Alex Gopher is mainly DJ and mixer. For the track Time there were additionally various vocals and a guitar solo recorded. Engineers on this recording were these two guys (quite famous producers and studio owners - Applehead Studios).
So you would leave it without further details, probably. This is the release:
My problem is that on that release I can’t see the engineer credits as there are two pages of edits open. So I look only at the first track.
If he is Engineering the vocals, then it is clear he is Engineer [vocals]. I would only break down the multiple specific types if he wasn’t looking after one category. For example, if someone else Engineered the background vocals, then I would specify which ones he was dealing with separately.
If he engineered BOTH vocals and instruments, then I leave him as Engineer.
I only add the separate tasks if separate people are doing them.
Vocals alone can’t be selected for engineer and that-like. Only a sub-category for vocals.
It was only guitar and not other instruments, performed by Alex Gopher. So it would be reasonable to credit Engineer [guitar], but the corresponding Engineer [vocals] is not possible, isn’t it?
Personally, given the absense of the exactly appropriate option, I would simply pick one of the options (maybe “other vocals”) and set the credited as field to “vocals”. I suppose you could add multiple relationships for each sub-type if you know it applies but that seems like overkill.
I agree it would be ideal if we could indicate generic “vocals” like we can for performer relations.
That’s it! That will do the job. Other vocals can be “credited as”. Thanks!
Please add an annotation about it at least, so we can maybe find it eventually once we’re able to select just “vocals” and fix it 
Should I add the original credits or something like that: (?)
Engineer credits for track 1 should be:
Engineer [vocals]
(not possible yet due to interface restrictions)
What you wrote sounds about right. You could add a “Sub-Optimal Credits” header, but I think that stopped being a mandatory thing we do like in 2010 
So as you wish with that (it sometimes does make things easier to find though).