Should I leave it in for a month, for all washes and rinses, or just put the basket in for the final rinse each cycle? What does it do when there's also soap?
It was designed to stay in for all phases. Vintage have quick cycles so it should be ok. Long modern cycles would cause it to dissipate in a cycle or two lol.
In certain dishwashers presence of rinse agent in all cycles can cause frothing issues. This is why some modern machines advise not to use solid rinse agents but the built in dispenser.
Know my GE Mobile Maid does not like froth created by Cascade "Complete" detergent)that contains rinse agent. If one cannot find the source of the rinse aid dispenser leak will not refill once empty but purchase solid rinse agent and place it in the tub during before the final two or last rinse. May just skip that and simply pop the lid and squirt a bit of Jet Dry liquid into the tub.