What if the aliens from V arvived on Earth durring the time of the TV show Enterprise, say 6 months after the Xindi tried to blow up Earth.
Are we talking 6 months after the first attack, or six months after the Xindi weapon is finally destroyed and the sphere-makers are defeated? That will make a big difference in how much force Star Fleet is able to bring to bear (there will be several more upgraded NX class vessels in the latter scenario, for example). However, I don't think it would make much difference in the end.
Technologically, the Sirians (the aliens from V) are much less advanced than Earth in the 22nd Century. According to a tech manual I found
here, basically the fastest ship they have (the Mothership) can travel at approximately twice the speed of light (approximately Warp 1.1). Starfleet vessels would literally run rings around them. They are powered by primitive fusion reactors. The mother ships also carry fighters and other support vessels, but these all operate in the sublight speed range.
Their weapons are also primitive...lasers of various kinds and power. One type of ship does carry a particle beam weapon similar to a Phase Cannon which is capable of destroying a small moon, but the ship itself is only capable of sublight speed. They also don't seem to have proper shields. All in all, the tech appears advanced enough to overwhelm early 21st century earth with ease, but not mid-to-late 22nd century earth.
I doubt they will even get into the Sol System, much less Earth orbit.