Fixes provided in routing for IFR flight plans - VFR waypoints okay?

I can't find any guidelines on the type of fixes or waypoints to be
included in the routing description of an IFR flight plan. I presume
you can use things like VORs and (?) airports, as well as airways and
anything indicated on IFR charts or procedures. But can you also use
VPxxx waypoints or other VFR fixes?

The reason for asking is that I'm wondering if a VFR flight plan with
detailed routing that lists specific waypoints and fixes can be easily
converted to IFR without changing the routing, should weather or other
factors mandate a switch to IFR for the flight.

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