  The discovery of Standard-Model like Higgs at 125 GeV may raise more
questions than the answers it provides. In particular, the hierarchy problem
remains unsolved, and the Standard Model Higgs quartic self-coupling becomes
negative below the Planck scale, necessitating new physics beyond the Standard
Model. In this work we investigate a popular scenario, extensions of the
Standard Model with vector-like fermion fields, such as the ones present in
models with extra dimensions or in Higgs composite models, using a model
independent approach. Since fermions decrease the Higgs quartic coupling at
high energies, only exacerbating the self-coupling problem, we introduce first
an additional scalar, which by itself is enough to overcome the vacuum
stability limit, and then explore the effects of vector-like fermions in
singlet, doublet and triplet representations. For each model, we identify the
allowed fermion masses and mixing angles with the third family fermions
required to satisfy the vacuum stability condition, and compare different
representations. Allowed fermion masses emerge at around 1 TeV, raising hope
that these will be found at the LHC. We also examine corrections to oblique
parameters S and T from additional scalar and vector-like quarks which also
impose constraints on mixing and mass splitting of both sectors., but these
restrictions are relatively weak compared to the vacuum stability.
