According to the properties of transition metal oxides :

1. Statement A is correct: All transition metals except scandium form MO oxides which are ionic.
2. Statement B is correct: The highest oxidation number in oxides coincides with the group number and is attained in $Sc_2O_3$ to $Mn_2O_7$.
3. Statement C is incorrect: In vanadium, there is a gradual change from the basic $V_2O_3$ to less basic $V_2O_4$ and to amphoteric $V_2O_5$. Thus, the basic character decreases, not increases.
4. Statement D is incorrect: $V_2O_4$ dissolves in acids to give $VO^{2+}$ salts, not $VO_4^{3-}$.
5. Statement E is correct: The well-characterised CrO is basic, but $Cr_2O_3$ is amphoteric.

Therefore, the incorrect statements are C and D.