How a principal bundle and the associated vector bundle determine each other It seems to me that given a vector bundle, the associated principal bundle is univocally determined. In fact one has to construct a principal bundle given the base, the fibre (the group $G$ in which the transition functions of the vector bundle take values) and a local trivialization whose associated transition functions satisfy the cocycle condition.
On the other hand, it seems to me that given a principal bundle, the associated vector bundle is far from unique: first one has to specify what is the vector space $V$ constituting the typical fibre, second one has to give a representation of $G$ on $V$. Even if the principal bundle is nontrivial, by taking the trivial representation the associated vector bundle is trivial.
If what I say is correct, why is the terminology "\emph{the} associated vector bundle" so widely use when there is no such an object, even if the vector space itself is specified?
 A: Given a topological space $X$ and a natural number $n$, to each rank $n$ real vector bundle $E\to X$ corresponds a principal $GL(n,\mathbb R)$-bundle $P(E)\to X$, called the frame bundle of $E$; the fiber  $P(E)_x$ over a point $x\in X$ is the set of linear isomorphisms $p:\mathbb R^n\to E_x$. The group $GL(n,\mathbb R)$ acts on $P(E)$ on the right by $g: p\mapsto p\circ g$. 
In the other direction, to a given a prinicipal $GL(n,\mathbb R)$-bundle $P\to X$ associate the vector bundle $E(P)=P\times\mathbb R^n/G$, where  $G$ acts on $P\times \mathbb R^n$ diagonaly by $(p,v)\mapsto (pg,g^{-1}v)$. 
You need to verify many small (and easy) details for $P(E)$ and $E(P)$ to be precisly and well defined. The upshot is that the maps $E\mapsto P(E)$ and $P\mapsto E(P)$ induce a  natural bijection between the set of equivalence classes of rank $n$ real vector bundles over $X$  and the set of equivalence classes of principal $GL(n,\mathbb R)$-bundles over $X$.  
You can modify the above easily to complex vector bundles and manifolds. 
So  we see that vector bundles (real or complex) correspond to  principal $G$-bundles,  where $G$ is the general linear group (real or complex). You can extend the correpondence to include other groups $G$, by adding more structure on the vector bundle (orientation, euclidean metric etc).
This may give the impression that working with vector bundles or principal bundles is "basicaly the same", perhaps a matter of taste. This is not the case, and both concepts are essential. But this will take too long to explain, perhaps the subject of a seperate question (sorry).   
