No, no, no. Combo vitamins like ADEK are useless for us. They sound perfect, but contain so little of each component as to be useless, and even potentially dangerous in that they make you think you are getting what you need when you aren't. You need the separate "dry" D, A, and K for sure. Your calcium has to be calcium citrate (not carbonate), which you can get cheap at Costco, just make sure you get the right one as they have both. Multi for sure. Iron for sure. You might, or might not, need B12 (I don't, but others do), but that's more of a gastric bypass problem than a DS problem.
Don't waste your money on fancy pants bariatric products. They are NOT designed for someone with a DS. You can buy almost everything you need between Costco and Amazon.