intros p H'; unfold FtoRradix, FtoR in |- *; simpl in |- *.
replace (1 * powerRZ radix (Zpred (Fexp p)))%R with
(powerRZ radix (Zpred (Fexp p))); [ idtac | ring ].
pattern (Fexp p) at 2 in |- *; replace (Fexp p) with (Zsucc (Zpred (Fexp p)));
[ rewrite powerRZ_Zs; auto with real zarith
| unfold Zsucc, Zpred in |- *; ring ].
repeat rewrite <- Rmult_assoc.
apply Rmult_le_compat_r; auto with real arith.
rewrite <- Rmult_IZR; apply Rle_IZR.
rewrite <- (Zabs_eq radix); auto with zarith.
rewrite <- Zabs_Zmult; rewrite Zmult_comm; auto with float.
Qed.

Theorem vNumbMoreThanOne : (1 < Zpos (vNum b))%Z.
replace 1%Z with (Z_of_nat 1); [ idtac | simpl in |- *; auto ].