Let $E \to X$ be an oriented vector bundle over a CW complex $X$. Suppose that the second Stiefel-Whitney class $w\_2(E)=0$, then $E$ admits a spin structure. In terms of homotopy theory this means that the classifying map $X \to BSO(n)$ lifts to $X \to BSpin(n)$. Does this mean that the homotopy classes $[X,BSpin(n)]$ classify all oriented vector bundles with $w\_2(E)=0$? And can it be that there is more than one lift? 


What puzzles me also is that the universal bundle over $BSpin(n)$ has not the same rank as that of $BSO(n)$. And therefore the pulled back universal bundle cannot be of same rank as $E$, therefore not isomorphic. How is it then possible that $[X,BSpin(n)]$ classifies vector bundles in $[M,BSO(n)]$ with $w\_2(E)=0$?



Understanding spinnable vector bundles with classifying spaces
Let me just expand what people written in the comments. 
Given a fibration $F \to E \to B$ of pointed spaces and $X$ be a space with a map 
$f: X \to E$. Then, the space of lifts $f' : X \to F$ is equivalent to the space of null-homotopies of the composition $X \to E \to B$.  

Choosing 
$F = BSpin\_n$, $E = BSO\_n$, $B = K(\mathbb{Z}/2,2)$ 
we see that the space of lifts of a map 
$f : X \to BSO\_n$ to $BSpin\_n$ is precisely the space of null-homotopies of 
the map $w\_2(f) : X \to K(\mathbb{Z}/2,2)$. 
If this space is not empty, then the map $w\_2(f)$ is null homotopic, and so we can choose a given null-homotopy $H$. Then, $H$ can be used to identify the space of null-homotopies with the space of null-homotopies of the constant map, which is $Hom(X,\Omega(K(\mathbb{Z}/2,2))) \cong Hom(X,K(\mathbb{Z}/2,1))$. 
Algebraically, this correspond to choosing a given $\alpha \in C^1(X,\mathbb{Z}/2)$ such that $d\alpha = w\_2(f)$. Then, every other such $\alpha'$ gives a 1-cocycle $\alpha - \alpha' \in Z^1(X,\mathbb{Z}/2)$.  

Anyway, without choosing the homotopy $H$, we just have an action of 
the "group" ($\mathbb{E}\_1$-space) $Hom(X,\Omega K(\mathbb{Z}/2,1))$ on the 
space of null-homotopies of $f$, and this action is a torsor in the sense that
the action map 
$$Hom(X,\Omega K(\mathbb{Z}/2,2)) \times NH(f) \to NH(f) \times NH(f)$$, 
$(T,H) \mapsto (T \square H,T)$ 
is a homotopy equivalence, where here $NH(f)$ stands for the space of null-homotopies of $f$.  

In particular, for every choice of point $H \in NH(f)$ we get a homotopy equivalence as stated above. 


Now all the results about cohomology are obtained by taking $\pi\_0$ from the topological fact above. 


