I believe that you are referring to the branded meal replacement product named Soylent. Yes, it is vegan. The company describes it as a ‘plant-based, complete meal’. So, it should be vegan. Interestingly, while I was doing research to answer this question, I came across another vegan product called Soylent. That is a combination of lentil and soya used as a meat substitute in an old novel titled ‘Make Room! Make Room!’. The novel is adapted to a film named ‘Soylent Green’.
 
The writer of the novel, Harry Harrison coined the name soylent. It is easy to confuse the meal replacement brand with the fictional food item, especially because both are related to veganism. I do not think the brand name just happens to be the same as the food item. I think that the latter is named after the former.