The FAA reg apparently requires a minimum of 1.23 Vso. I've read it, don't have the time to search for it right now.
737-800 Flaps 40 uses 1.25 Vref. Don't take my word for it, do the math vs. the stall speeds in your performance manual. I originally saw the 1.25 Vso explaination on pprune and verified it using our performance manuals.
Stall speeds Flaps 30 vs. Flaps 40 is 2 kts. Vref difference is 7 kts. The 1.3 Vso for Flaps 30 vs 1.25 Vso for Flaps 40 explains the Vref difference.