  Determining the precise value of the tangential component of the velocity of
M31 is a non trivial astrophysical issue, that relies on complicated modeling.
This has recently lead to con- flicting estimates, obtained by several groups
that used different methodologies and assump- tions. This letter addresses the
issue by computing a Bayesian posterior distribution function of this quantity,
in order to measure the compatibility of those estimates with LambdaCDM. This
is achieved using an ensemble of local group (LG) look-alikes collected from a
set of Con- strained Simulations (CSs) of the local Universe, and a standard
unconstrained LambdaCDM. The latter allows us to build a control sample of
LG-like pairs and to single out the influence of the environment in our
results. We find that neither estimate is at odds with LambdaCDM; how- ever,
whereas CSs favour higher values of vtan , the reverse is true for estimates
based on LG samples gathered from unconstrained simulations, overlooking the
environmental element
