You should maintain the salt and temp for at least one full week (I do two for safety sake) after the ich is no longer visible. It still has 1 life cycle to go through before you can kill it. So in other words, you haven't killed the ich yet, it just dropped off of your fish and started the reproductive stage. Gravel vaccuming will help during this stage but won't garantee eradication. You are winning the battle, but need to keep fighting. ICH is only succeptable when it hatches out of the substrate and goes looking for your fish. otherwise it is difficult if not impossible to kill.
A q tank can be anything fish safe that holds water, and you have a filter and heater for. For me personally, I would get a 10 g and cycle it then keep some snails and food in it when not in use. the food and the snail waste will maintain the bacteria, and then when you need the q-tank it's ready. Otherwise just set it up when needed, and be prepared to rob large volumes of filter media from your main tank to cycle with.
Dave