I am also considering a pair of Vass in the F-last. I wear the following sizes in other brands:
9.5D US in C&J for BB 341 last (probably corresponds to 8.5E UK, but not sure)
8.5E UK in C&J 348 and 365 lasts
42 in MMM boots
42 in Fiorentini and Baker boots
All of the above fit me very well. The fore-foot in the C&J 365 is roomy, as I assume it is meant to be, as it is a country boot.
Any advice would be appreciated.