Cushioncovers
Vitamin A. Only found in animal produce. It's possible to get some via betacarotene, but many people are very poor converters of it and end up deficient.
Vitamin B12. Again only found in animal produce. Can be produced synthetically and added to supplements and fortified food, but the fact that it's only found in animal products indicates we're evolved to eat meat primarily. Vegans allege that its ultimate source is the soil... I don't see many of them eating veg with soil still caked on, they eat washed veg like the rest of us 
Also, that's contingent on there being healthy soil in the first place, which grain based soils destroy... you need to have animals making their "deposits" in the farming system to have healthy top soils. And no, veganic manure systems aren't 1% close to feeding the human population. Deffo not found in seaweeds... it's a pseudo form of B12 that blocks the uptake of the genuine form and leaves you more deficient. Vitamin B12 deficiency is "silent", you're often not aware of it until it's too late and you have entrenched fatigue and unshiftable depression/mental illnesses.
Vitamin D3. Found in grass fed animal products and oily fish. And sunlight on the skin from April to Sept (UK). Actually all people in the UK & similar latitudes should take a supplement, unless you happen to be eating like 1/2 kg of wild salmon a day. It's taken from lanolin in wool, so not vegan (unless genetically modified)
Vitamin K2. Again, only found in animal produce, apart from an obscure Japanese food made from fermented soy beans called nattō. The Japanese themselves have a marmite-y love/hate relationship with it... only a miniscule numbers of westerners will find a taste for it. Vitamin K2 is crucial for working together with vitamin D to shepherd the calcium in the body to where it needs to be... ie in the teeth and bones and not in the soft tissue where it causes heart and vascular disease. Many longer terms vegans have found they have crumbling bones (eg Lierre Keith spine problems) and bad tooth decay, no matter how meticulous they are at dental hygiene. Please please, at least once a month eat some liver, gouda cheese or grass fed butter to get some in your diet. Or at least get some nattō-based supplements to prevent your bones and teeth falling apart.
Heme iron. Much more bioavailable than the plant forms, which come packaged with antinutrients such as oxalates in spinach which block the uptake of the minerals within.
DHA. Only found sufficient amounts in wild caught oily fish and grass fed meat and dairy. There are other Omega 3 fatty acids, but the body is very poor at converting them to DHA. DHA is what your brain needs to build itself and function. Also, the ratio of Omega 3 : Omega 6 is something like 1 : 20 in grain based diets. We need it to be close to 1 : 1 in order to be truly healthy and avoid inflammation. It's impossible to find that on a vegan diet (a few obscure algae-based DHA supplements notwithstanding... they put a few more mg in the diet, but doesn't address the ratio needed). Crucial for brain development in children.
Creatine. Can be produced in small amounts in the liver, so not strictly essential. But not sufficient to provide the optimum amount of energy to muscles. Only found in meat (and manufactured supplements).
Many other nutrients and co-factors that researchers haven't even scratched the surface of that are essential or highly beneficial to health. Vitamin K2 was only discovered relatively recently, although its absence was observed several decades ago... scientists couldn't work out for years what the missing factor was in the diet.