  The Standard Model fermion sector is enlarged by either one light singlet
vector-like down-type quark or one light vector-like lepton doublet, which
might be accommodated within a five-dimensional representation of SU(5). At low
energies the inclusion of these states affects precisely measured observables
in flavor physics, as well as electroweak precision measurements. These
experimental results strongly constrain couplings of vector-like states to the
Standard Model particles. Having these bounds, we investigate the impact of
vector-like fermions on the mass matrices for down-type quarks and charged
leptons in an SU(5) setting. We find that unitary transformations relating an
arbitrary flavor basis to the mass eigenstate basis depend only on three free
parameters. Then we discuss the parameter space constrained by low-energy data
assuming vector-like quark and vector-like lepton masses to be 800 GeV and 400
GeV, respectively. We demonstrate that these two scenarios generate unique
patterns for relevant proton decay widths. A further improvement of
experimental bounds on proton decay modes would thus differentiate the allowed
parameter space. We finally present two full-fledged SU(5) models that allow
for gauge coupling unification with light vector-like fermions under
consideration and discuss their viability.
