How do VAC ratings of flyback transformers translate to VDC ratings of flyback converters?
I think you are confused about transformers in general and flyback transformers in particular.  Transformers are not fed with DC typically.  While you could use half-waved rectified &quot;DC&quot; (and it's not really DC) but full-wave rectified DC would do nothing but generate heat in the primary winding of the transformer.  Without AC you get no changing magnetic field and you get nothing coupled into the secondary.
Flyback transformers are usually fed high-frequency AC in order to be able to get an efficient HV output with a relatively small transformer.
**** I edited your question to try to make it more clear. ****
If you are talking specifically about flyback transformer circuits, then I agree.  The most common driver takes a DC voltage and switches it to feed the primary of the transformer.  Often this is a kHz-range signal.
These are used for two reasons.  First the core needed for a 15 kHz flyback is much smaller than one needed for a 60 Hz flyback.  Second, it's often desirable to locate the driving frequency above the typical range of human hearing since these things can often be heard and can be quite annoying.