I'm normally producing webinars with a number of PowerPoints - mixed into the production.
In addition to include the PowerPoint in the mixed output I also need to send the the current state of the presentation to a monitor near the presenter.

My first go was to use a stand alone PC with PowerPoint. HDMI-out goes to a HDMI splitter which sent one signal to he monitor and another signal to vMix - either using NDI og a slot in the capture card
Disadvantages: Extra PC, I can't switch between multiple presentations in vMix

I have also tried to include all the presentation as inputs.This gives me full control, but how can I sent the PowerPoint input to a external monitor?
I know I can sent an input to NDI or an external output, but if you have like 10 PowerPoints, I guess you constantly have to change the source of the output to the appropriate PowerPoint presentations.

Any help is appreciated.

As dmwkr says, you can setup shortcuts to change the input sent to your external output, but that takes time to set up for each input.

If I needed to switch between several sources in a hardware setup, I would probably use a small video switcher for the powerpoint PCs and distribute the output to the main vision mixer and the stage monitors. In vMix, the equivalent is using an extra Mix input. By switching all your powerpoint sources in a Mix, you can easily setup your PIPs, external outputs, etc with the Mix as the source. Of course, this only works if you have the 4K or Pro editions of vMix.

My set up depends on the way the PPTX was created. If it's slide, slide, slide...no "building" slides (eg: no new bullet point on each next click), I make them an input in vMix.

If presenter has building slides, my preference is to connect from their HDMI out port, using 1 to 2 splitter if need be so they can still have it projected (or confidence monitor like you mentioned). That way, as with one of my clients who places a sponsor highlight video in their slide deck, I still have full video/audio to output, as well as if the presenter is demoing a website/software on the screen. Also helps with the "Oh, I changed the slides at breakfast, my bad!" ugliness when the streamed to view doesn't have the updated content.

Also: If you only feed a confidence monitor, just plug up to your HDMI output and turn on Fullscreen function to send Program frame there.

If you have both projector and confidence monitor, I've further split the projector feed off the second laptop (where I take my input to another capture device) to 2 physcial projectors and the confidence monitor in front of the stage with no issues of video quality.