The way VGT works normally is that the main floor is at 100% difficulty and each tower increases the difficulty by 10%, up to 190% in tower 9. Then normally H in VGT has 75 base damage.
With VT all the towers and the base floor are set to a flat 160% difficulty which makes the later towers easy and the early ones much more difficult, as I'm sure you saw (once you hit level 10 the rest of the towers were pretty easy). Additionally VT makes H's base damage 120, which goes up to 180 once he berserks, as opposed to 75 -> 120. That boss fight is much more difficult, especially for a Vampire who can just straight up use the first half of H as a blood cow without VT.