HI tmaleski and welcome to the community!
Can I create a VM from MDT using Workstation 15?
Yes you definitely can.
If so, is there a way to speed up the process?
18 hours seems an awfully long time to be deploying a machine and only get to 18% so I'm sure there is a way to speed this up. What version of MDT are you using? What OS is being deployed? What is the host hardware you are running the VMs from?
Kind regards.