codejedi,
1) Bottom line is that the prettier configuration file would be the way to go, CodeMix will not be able to run an external command to format the files.
2) Have you checked the settings under Preferences > Formatter > Vue and Preferences > Extensions > Vue > Format Default Formatter – are these the options you are talking about, and have already tweaked?
3) Any difference in behavior if you format the file manually (i.e. not on save)?
4) When creating the project you could choose to use dedicated configuration files (you could do this for a new project and copy the files over) this should create files in the project that the IDE formatter should respect.
If not too much trouble, would it be possible to send us a sample project that exhibits this problem? Will help ensure we’re on the same page and can work on a fix, if required, ASAP.
Thanks!