For example, I really, really like this "stark silhouette" dress (pictured right, $198). It is a cotton and polyester blend with modal lining. At 39.5 inches in length, it likely hits a bit below the knee, which is perfect for the office. It would look great with a black cardigan or a sweater jacket, and with its cap sleeves and high neckline in the front and back (the neck line is very high in the back), it might be office appropriate even without the cardigan or jacket. Regardless, I think the dress definitely is appropriate for a daytime office party.
For something more formal, I love this "gregarious" black dress (pictured left, $188). It is made from cotton, acetate, and metallic thread. At 36.5 inches in length, it probably hits just at or under the knee. I also am impressed that the sash is removable: two dresses in one!
I also enjoy this little black dress ($188; polyester, rayon, spandex, acetate) and this more expensive, though definitely office appropriate black dress ($318; rayon, nylon, spandex). For something patterned, there is this "parchment flurry" dress decorated in a conservative black and tan leafy print ($168; cotton). Good job with the vegan dresses Anthropologie!