. . . . convert an iso file to a format VPX will recognize . . .
You cannot convert an iso file to a format VPX can load, however you can copy the video files from it.
If you are using Windows 10 - open Explorer right click the iso file and select Mount - if Mount is not there, select Open with and then select Windows Explorer from the list - this will create a virtual drive which you can then access the same way as any other drive.
You can then browse the disc and copy the files to a folder on the PC - if it is a DVD then copy all the files in the VIDEO_TS folder, if it is a Blu-ray disk then you will find the video files in BDMV, Stream folder.
In both cases one or more of the VOBs or m2ts files will be the menu and chapter video files, in the case of the DVD files, in VPX browse to the folder they are in on your PC and add the icon you see - this should load all the VOB files in the correct sequence.
Perhaps an even easier method than that suggested by John EB. An .iso file is actually a zipped file so using any unzipping software on it, even the free 7Zip, will unzip it and allow you to access any or all of the constituent files. Then proceed as John suggests.