  We reconsider the construction of solitons by dressing transformations in the
sine-Gordon model. We show that the $N$-soliton solutions are in the orbit of
the vacuum, and we identify the elements in the dressing group which allow us
to built the $N$-soliton solutions from the vacuum solution. The dressed
$\tau$-functions can be computed in two different ways~: either using adjoint
actions in the affine Lie algebra $\hat {sl_2}$, and this gives the relation
with the B\"acklund transformations, or using the level one representations of
the affine Lie algebra $\widehat{sl_2}$, and this directly gives the formulae
for the $\tau$-functions in terms of vertex operators.
