Hello Justin
Yes, these products are very similar as far as what they do. They are indeed products that conform to the Software Defined Infrastructure model where network management is implemented using a centralized Network Management System (NMS). Of course vManage is used for Viptela devices while DNA is used for Cisco.
Both SD-Access and SD-WAN are related in that they are technologies that use software as a method of centralized network management. One deals with the Access layer of a network while the other deals with the WAN. SD-anything is an umbrella term under which both SD-Access and SD-WAN fall under. Just like networking is an umbrella term under which access and WAN networks fall under. It’s the same relation but under different architectures.
So simply put, yes, the difference is that you don’t use SD-access to connect to remote sites, but SD-WAN instead.
I hope this has been helpful!
Laz