I have a similar monitor from Dell, the S2340L (23"), and it doesn't have a VESA mount as well. A company called HumanCentric sells VESA adapters for these monitors and I've been using it for over a year with no issues. They price gouge on those adapters, I think mine was $25 at the time, but it still comes out pretty cheap in total. If you go this route, make sure the adapter matches the monitor model. Just my 2 cents.
Dell monitors are great btw. I just picked up a U3415W ultrawide for my new setup and it's amazing.