  We investigate the standard model (SM) with the extension of a charged scalar
having fractional electromagnetic charge of $-1/3$ unit and with lepton and
baryon number violating couplings at tree level. Without directly taking part
in the electro-weak (EW) symmetry breaking, this scalar can affect stability of
the EW vacuum via loop effects. The impact of such a scalar, i.e., a leptoquark
on the perturbativity of SM dimensionless couplings as well as on new physics
couplings has been studied at two-loop order. The vacuum stability of the Higgs
potential is checked using one-loop renormalization group (RG) improved
effective potential approach with two-loop beta function for all the couplings.
From the stability analysis various bounds are drawn on parameter space by
identifying the region corresponding to metastability and stability of the EW
vacuum. Later we also address the Higgs mass fine-tuning issue via Veltman
condition and the presence of such scalar increases the scale up to which the
theory can be considered as reasonably fine-tuned. All these constraints give a
very predictive parameter space for leptoquark couplings which can be tested at
present and future colliders. Especially, a leptoquark with mass
$\mathcal{O}\,$(TeV) can give rise to lepton-quark flavor violating signatures
via decaying into the $t\, \tau$ channel at tree level, which can be tested at
the LHC or future colliders.
