Hhm, it would seem you might have some confusion about the PNG format. There's no such thing as a "vector PNG". PNG is a raster format. So that would seem to be a dead giveaway as the source of at least one problem.
Actually, yes you CAN distinguish between raster images and vector content. Let me introduce you to the status bar! When the Selection tool is enabled, and an object is selected, the status bar tells you what kind of object it is. So if you select something, and the status bar says "Image....." that is a PNG. If it says "Path" it is vector. If it says "Group" it might or might not be vector. You'd have to either Ungroup, to be able to select each object; or you can enter the group, and select individual objects without ungrouping. Once you've identified a selection as a group, double-click on it, and you will be inside the group, and able to select individual objects.
Depending on what's in the PNG, you might be able to auto trace using Path menu > Trace Bitmap, which would convert it to paths. Not all raster images can be converted to vector with acceptable quality, but often they can. Or if they PNGs are some kind of symbol or other text, you could recreate with Inkscape easily.
If you can show us an SVG file, we could give you an idea about whether the PNGs could be successfully auto-traced.
