  Unification at M_{GUT}\sim 3\times 10^{16} GeV of the three Standard Model
(SM) gauge couplings can be achieved by postulating the existence of a pair of
vectorlike fermions carrying SM charges and masses of order 300 GeV -- 1 TeV.
The presence of these fermions significantly modifies the vacuum stability and
perturbativity bounds on the mass of the SM Higgs boson. The new vacuum
stability bound in this extended SM is estimated to be 117 GeV, to be compared
with the SM prediction of about 128 GeV. An upper bound of 190 GeV is obtained
based on perturbativity arguments. The impact on these predictions of type I
seesaw physics is also discussed. The discovery of a relatively `light' Higgs
boson with mass \sim 117 GeV could signal the presence of new vectorlike
fermions within reach of the LHC.
