You are right that under sail the lift serves no purpose. You''re fine to leave it a bit slack, so it is not fighting against the mainsheet or the vang. You just have to remember to re-tension it before you drop the mainsail.
You don''t need to replace it with a solid vang, but I prefer a solid vang (not the boomkicker) for the sole purpose of getting rid of the lift and the required adjustments when dropping the main. I disagree with the previous post in that I would not be worried about leaving the weight of the boom, even when tensioned by the mainsheet, against a solid vang.