"I think vacuum-operated wipers are still used on buses."
Generally, busses are diesel powered. diesel engines lack a throttle plate, and therefore have no manifold vacuum. to make up for this, they have a vacuum pump which is driven by the accessory belt. the vacuum pump is able to generate vacuum at any RPM, so if some buses do indeed use vacuum operated wipers, they should work just as good as their electric counterparts and not have the issue of not working when during acceleration like what happens vacuum operated accessories on a gas engine.
