For the extent of my soprano career, conductors and choir directors have continually asked me to sing without vibrato in choral numbers–for the sake of better tuning and blend, of course. This has been not just a challenge for me, but actually has impeded my singing. By the end of the concert I would be too hoarse to phonate even the most conversational pitches.
My take? Vibrato is natural, healthy, and actually makes a choir blend and tune better than without it, contrary to what some directors believe.
This is a great article explaining exactly what I’m talking about: