strength

The ability to use superhuman strength is one of the characteristics that set vampires apart. Their physical strength is seen in stark contrast to the fragility humans. Vampires can move seemingly heavy loads with ease, and without effort. Also, a vampire’s strength is yet intensified when the vampire is brand new. Newborn vampires are immensely strong for their first year of life—even more so than a vampire that isn’t a newborn. This is also a product of the excess blood left in the body.
This implies that that a vampire’s strength and energy levels are refueled by hunting, although hunting makes a minimal difference to the physical strength levels of a vampire, and for a limited amount of time. Even a thirsty vampire is capable of great feats of physical strength. Although, a vampire’s strength is very much dependent on the individual—each vampire has their own personal range. But in the end, skill is going to count more than strength, so an experienced vampire could still outwit a newborn in a fight.  
A vampire’s diet does not have a massive impact on the levels of strength—human blood makes them stronger than animal blood, but only fractionally. Feasting on big game makes them stronger than the "weaker" blood of herd animals.  
Ultimately, physical strength is always going to come down to the individual vampire, rather than secondary factors like their diet. Strength, of course, isn’t always physical. It is impossible for a vampire to die through physical starvation, but the lack of blood intake does alter a vampire’s mental state. A vampire that lacks blood intake will grow very hungry and very weak. When vampires are thirsty, they are less capable of acting with rational thought, and are therefore more likely to act through their natural animal instincts. Obviously, the blood lust of the vampires is a natural urge, and is hard to resist, and even more so while they are thirsty. Their willpower weakens when their thirst is wild.