  Consider an o-minimal structure on the real field. Let $M$ be a definable
$C^r$ manifold, where $r$ is a nonnegative integer. We first demonstrate an
equivalence of the category of definable $C^r$ vector bundles over $M$ with the
category of finitely generated projective modules over the ring
$C_{\text{df}}^r(M)$. Here, the notation $C_{\text{df}}^r(M)$ denotes the ring
of definable $C^r$ functions on $M$. We also show an equivalence of the
category of definable $C^r$ bilinear spaces over $M$ with the category of
bilinear spaces over the ring $C_{\text{df}}^r(M)$. The main theorems of this
paper are homotopy theorems for definable $C^r$ vector bundles and definable
$C^r$ bilinear spaces over $M$. As an application, we show that the
Grothendieck rings $K_0(C_{\text{df}}^r(M))$, $K_0(C_{\text{df}}^0(M))$ and the
Witt ring $W(C_{\text{df}}^r(M))$ are all isomorphic.
