I've found how to cut vinyl with inkscape - finally.

Now, Inkscape has updated and my previous method doesn't work. So I downgraded and did it the way I did before.

Select 'Print' , then in the box that comes up paste the following:
| /home/bruce/hpgl/print-to-cutter.sh

that is a nice shell script for using a knife plotter

The newest Inkscape uses the system printer settings, so the little box no longer comes up.

How do I create a "Pipe" printer?

I know nothing about this, but you might like to look at using the command line.