The vocal vibrato isn't any sort of issue when you're doing the bvox - they just blend the differences into a nice chorusing effect.
You could even it out on the single vocal section with software/plugins inside your DAW or you could double track all the vox to achieve the same effect as with the bvox.
What the "warble" does do is suggest age in the singer.
The song's a goodun..


Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe