If you want to turn an existing custom female shape into a futa, there are some extra steps:
Save the custom figure's shape as a morph-preset.
Create a basic futa like in the tutorial and don't forget to click those "use female morphs on males".
Save, because VaM has no "undo" function.
Now use the morph-preset on the futa model.
Nevertheless, you most likely have to do some refinements.
One step more complicated, if this will not work for some reasons:
Create a merged morph of your custom female with the MorphMerger plugin and use this morph on the futa.