The Airfix figures (dating from the 1960s) are polythene, not vinyl, whilst the Revell figures are (fairly soft) styrene. Once washed as previously described, there should be no problem painting them and, if handled carefully, the paint should adhere. However, natural greases from fingers, will tend to cause the paint to lift, so it's a good idea to leave each figure as long as possible before placing them into the diorama.
Also, ensure that the (enamel) paint is thoroughly stirred before painting the figures, in order to minimise the possibility of the pigment not being fully integrated with the carrier oil - this would cause the paint to lift, or not adhere, especially on the Airfix figures. I still have some of the latter, painted around 30 years ago, which have hardened and easily resist chipping and flaking.
Have a look at my Halifax build, as I have used exactly the same figure sets, and, in the case of the Revell figures, these are Luftwaffe converted to represent RAF personnel.