I actually despise the VDI accronym because it so vague in how it can be used. Typically you can provide a virtual desktop through two methods…
- Hosted desktop from a Windows Server - This is where you would need CALs, because all the clients connect to the Windows server, and get a desktop like experience. Through GPOs and theming, you can make it look like a normal Windows desktop. It has its pros and cons, like it can scale with limited resources, but at the same time, can suffer from a couple users potentially consuming a lot of compute if things go sideways.
2)Standalone Windows 10 desktop as a VM - Just pay for the windows licenses as you would for any Windows Desktop. Depending on the hypervisor you may have licensing costs for the hypervisor itself. This model isn’t necessarily as scalable and requires more hardware to scale out, but the advantage is, the users work in silos, so one or two users generally don’t cripple the performance if something goes sideways with their session.
Both of these methods require some way to connect the client to the VM. RDP works as kind of a 1:1 connection, and is built in, and would work for your scenario. Otherwise you’d have to consider something like Citrix, or VMware, or other, and these do have a licensing cost. These provide optimizations over RDP, as well as give additional functionality, but if you just want to run an app on a remote machine, they probably won’t be required.