M Ted has it right. If people want to blend or sing harmony, #1 goal has to be straight tone. Vibrato makes it dramatically more difficult to blend. Very rarely does any choral conductor actually ask for vibrato. Our choral group is officially off for the summer but I was recently in a little subset of the large group, providing the chorus for a performance of "La Boheme". Our conductor said he wanted a rich tone--including vibrato.   But he hastened to add that he recognized this was a dangerous request--since in virtually all other circumstances he wanted no vibrato.

Garrison Keillor had a skit in which various composers had the position of choir director/organist at a little church.   Gershwin was a failure since when they passed the collection plate, they tended to get responses like "I'll have a Manhattan".   Messaien was also one of the composers who didn't last. Reason:   he "believed in chance elements in music and we already had that with the older sopranos".