I don't know if this is any help, but I wear vari-focals all the time now (they're pretty good for me once I get used to them - I'm now on my 3rd pair and it takes just a short while) and the problem I've found with all safety glasses that I've tried is that they just don't fit comfortably over the top of my normal vari-focals (and I can't see much at all without my prescription specs). Even if they start off more or less fitting OK they soon slide down, etc. AND they steam up if wearing a mask.
So I tried this:
It cost roughly a tenner, plus another fiver for a spare 2nd visor. It is a bit "clunky" on your head, but quite comfortable when you (quickly) get used to it, and if wearing any sort of dust mask it's MUCH better than any safety glasses I've tried from the steaming up aspect. It is "Class B" BTW.
I posted that pic of Axi's version but I'm sure there are other brands. As usual I'm pretty satisfied with both the product and the Axi service - usual disclaimers.
One further point: I guess you have them in UK supermarkets too - I buy little sachets of "dampened" paper wipes specifically designed for cleaning glasses (in my case I buy from Aldi here) and a quick wipe over the visor before use not only reduces the little steaming up that does occur but also helps to stop dust sticking to the visor itself (no doubt due to static - the visor is plastic of some sort).
This is not the complete answer to the maidens prayer for me but the best I've tried so far. I have never heard of getting prescription glasses made for safety purposes though, so I may try that too (though I bet it'll be V expensive, just like everything else here too).
HTH
AES
Edit added:
Thanks to the poster who linked to the original thread. Very useful and now bookmarked - I seem to have missed that thread first time around somehow.
AES