Roy Smith wrote:

It is certainly legal to fly IFR without a VOR receiver, but it's pretty
much taken for granted that you've got one. It's certainly taken for
granted that you've got a chart!

The national airspace system is still predicated upon VOR being the primary,
non-radar en route navigation system. That will change, but it hasn't yet. The
section in part 91 that requires you to have equipment appropriate to the routes
to be flown presumes VOR equippage. No, it doesn't say that, because such is
the way of many FARs. You either comply with the obvious or you have to be armed
with all the countless policy documents with which the FAA operates.

For whatever it's worth, following is from Pargraph 813 of FAA Order 8260.19C,
Flight Procedures and Airspace, the gudiance material for instrument procedures
designers:

.. "Dual Minimums. Dual minimums, when authorized, shall be entered in boxes
constructed below the preprinted minimums section. Dual minimums shall not be
authorized unless a 60 foot operational advantage is obtained or a reduction in
visibility can be achieved. To avoid proliferation of dual minimums, all IFR
aircraft are assumed to have at least one VOR receiver. Dual minimums based on a
stepdown fix combined with local and remote altimeter settings could result in
four sets of minimums. However, only two sets of minimums shall be published on
the 8260 forms. The combinations authorized a minimums with and without a
stepdown fix; or minimums with local and remote altimeter settings."

This is available on the Summit Aviation Reference Library.