When: Whenever you can
When you absorb with the bucket you have momentary Invincibility and Sheik has intangibility on her Vanish, it is very safe to do this. If needed try to have Sheik's Vanish hit people away from G&W, G&W has some lag when putting away the bucket and can be punished.
Sheik and G&W are both good at keeping people off stage, in a 2v1 situation the 1 has a hard time stopping them from partially filling the bucket.
For Example:
Sheik is Fairing someone off stage, G&W can jump out there and bucket a Vanish (allowing Sheik to get back on stage and knock the other away) after G&W fills the bucket and then puts it away the off stage enemy should be close enough for Fire to hit them (from what I noticed this usually hits away from the stage) Assuming that Sheik did not bring the other person all the way to the other side of the stage a second Vanish should fill the Bucket.
Where:
I would say most stages are good for the Bucket Strategy. The wider the blast zones the better, since the bucket will likely kill if it lands and G&W is really light (3rd lightest).
These are just my thoughts on it
