Inhuman Strength adds +3 to Might for the purpose of lifting or breaking things. So a Might 5 mortal and a Might 2 vampire (with inhuman strength) will be able to deadlift about the same amount on average (barring differences in rolls, if appropriate) and will be able to break about as many boards when doing the karate-chop contest thingy. The vampire only gets +1 to grapple checks, so the mortal will actually be able to out-wrestle the vamp where 'skill' is the issue. The vamp, however, benefits from being able to inflict extra stress in grapples or strength-based attacks (the effectiveness of this will depend more on relative Weapons/Fists/Athletics than Might), so any time the mortal fails to defend against the vampire, it will hurt much more than the reverse.
Moving up to Supernatural Strength, a mortal has to rely on luck to compete with even the most wimpy monster in lifting (maximum 5 Might for the mortal, minimum 6 Might for a completely unpracticed monster). The mortal might be able to rely on skill in grapples, but unless his fighting skills outclass the monster, he'll likely soon be tenderized into a tasty snack by the monsters powerful blows.
That's my take, at least...