Do you have to use the vacuum? They have those telescoping duster things for that purpose. Something like the one shown below, but there are other styles.

Here is a link that might be useful: ceiling duster

I put the wand things from my tank and those from the shop vac, together and manage to reach to the top of the stair well.
I always save the extension things when a vacuum bites the dust.
Linda C

I use a paint pole extension rod. Duct tape my feather duster to the end. Works like a charm. Since I only do it when I can actually see the cobwebs it isn't much effort. My pole telescopes back down to a reasonable length for storage.

Gloria