This is a very late response, but there is no accepted answer as of yet, and none of the answer quite hit the mark.
Regarding the magical collision hypothesis, that smacks of being rather non-scientific. Scientists as well as Missourians are wont to say, "Show me!" Other than the fact that Venus's rotation is anomalous, what, exactly, is the evidence for a collision with enough oomph to create this anomalous rotation? Even more problematically, this collision hypothesis hand waves away the problem of Venus's thick atmosphere.
Part of the problem here is thinking that the current rotation rates and rotation axes of the terrestrial planets are somehow related to the initial angular momentum of protoplanetary disk from which the planets formed. That may well be the case for the two gas giants in the solar system, but it's not the case for the terrestrial planets. The primordial angular momenta of the terrestrial planets is not a conserved quantity thanks to external torques from the Sun, Jupiter, moons, and other planets. Mercury is in a 3:2 spin-orbit resonance. Mars has suffered chaotic variations in its rotational state due to perturbations from Jupiter. Whatever angular momenta those two planets had initially is long lost. The Earth's Moon has apparently stabilized the Earth's rotation axis, but has sapped its primordial rotation rate. So what about Venus?
Helder Velez's answer to me comes closest to the mark but misses some key points. Helder explicitly discounted Venus's thick atmosphere as playing a role. That Venus has a very thick atmosphere may well be a key part of the answer. Helder referenced the second of two papers published in Icarus on Venus's rotation by Correia and Laskar but did not the reference the first (or the similar Nature article by Correia and Laskar published couple of years prior to those Icarus articles), and Helder did not pay attention to the key point in the Correia and Laskar: Venus rotation is a natural consequence of Venus's thick atmosphere. No collision is needed.
A parsimonious explanation of Venus's rotational state would not need a magical gigantic impact that even more magically did not blow away Venus's primordial atmosphere. This parsimonious explanation is exactly what Correia and Laskar argue happened in their Nature paper and their two Icarus papers. Venus rotates the way it does because this is one of the four final states in which a collision-free terrestrial planet with a very thick atmosphere would rotate. Two of those final states have Venus rotating prograde, the other two, retrograde. The prograde rotational states are statistically unlikely compared to the retrograde rotational states. Venus's thick atmosphere plays a key role in determining Venus's final, stable rotational state.
Correia, A. C., & Laskar, J. (2001). The four final rotation states of Venus. Nature, 411(6839), 767-770.
Correia, A., Laskar, J., & de Surgy, O. N. (2003). Long-term evolution of the spin of Venus: I. theory. Icarus, 163(1), 1-23.
Correia, A., & Laskar, J. (2003). Long-term evolution of the spin of Venus: II. numerical simulations. Icarus, 163(1), 24-45.