The idea that Venus is somehow the founding deity of Rome is, to a large extent, a result of the Aeneid's success. Before Vergil wrote this poem, the story of Rome's origins were still up in the air.

Until then, Aeneas is viewed not as the ancestor of all Rome, but just of the gens Iulia (the Julian clan), to which Julius Caesar and Augustus Caesar belonged. The Julii seem to have made a connection with Iulus, son of Aeneas, as early as the 3rd century, but again, that's just for one family.

The most important Roman ancestor is Romulus, whose father is Mars, which is why Mars has a better claim to be the founding deity of Rome than Venus does. Even before Vergil, some authors connect Romulus with Aeneas (as Ennius seems to do, by making Romulus' mother Aeneas' daughter), but there are numerous other versions of the founding of Rome, many of which have nothing to do with Aeneas or his descendants.

So, it's really only when Vergil writes the Aeneid that Aeneas comes to be viewed as the founder of the Roman race, and it's hard to stress how big an innovation this is. Vergil is saying that Aeneas is the founder of a race, which is not the way ancient ideas of foundation really worked, and Vergil is constructing an idea of Romanness that predates Rome.

The popularity of this poem makes everything seem neat and canonical now, but it was far from that way at the time, and it was a combination of Vergil's poetic skill and the political power of the Iulii at the time. Because she was the founding divinity of the Iulii, she comes to take on a larger role when they become synonymous with Rome.

If you want to read some of the other stories of Rome's foundation, the early chapters of Plutarch's 'Life of Romulus' are good, as is the first book of Dionysius of Halicarnassus' 'Roman Antiquities.'

A solid overview of the Aeneas legend before Vergil can be found in Horsfall's 1987 "The Aeneas Legend from Homer to Virgil" in Roman Myth and Mythography (Bulletin of the Institute of Classical Studies Supplement 52), pp. 12ff.