knowing the right size of a shoe/boot in the different lasts is actually pretty easy...do enough of research (there should be plenty of advice in this threadthe intact the brand (in this case viberg) and ask for sizing advice and if you've made a choice, pay to play!
if that is too much to ask for, then this right here isn't really for ya...just my 0.02$!
I bought my first viberg in the wrong size, sold it and bought a different one (0.5 bigger in size, different last as 1st pair) and the fit is just perfect! then after that I've bought another pair (same size as previous, different last) and again the fit was perfect! so it actually just took me one false move and (after that) a lot of research, to finally know my exact size in viberg and their different lasts!
it's actually the same with dry denim that you just can't try in the store around the corner...do some research, some measuring, compare I other things you wear and voila, there you go!
I know it's pretty risky and some folks just don't want to spend that much money for the "wrong boot", but it is what it is...deal with it or get lost!