VS1 and D156 serve completely different purposes to me… VS1 is a silica based sealant that is designed to work both as a compliment to an existing coating, or as a stand alone sealant. On its own, VS1 will lasts several months, based on personal testing with my own vehicles. D156 is a spray detailer that has some synthetic polymers in it which can provide some short term protection and gloss. I like to use D156 as a drying aid to add lubrication while drying. It is extremely easy to use, and therefore adds no time to the washing & drying process for me. VS1 is something you can choose to use frequently due to the minimal time required to apply it, or I prefer to use it every 1-3 months for added protection on top of our coated vehicles.