I'm actually more afraid of a competent tau player than an competent IG one when I play guard. Railguns own face.
Vendettas are like cheaper broadsides (140 for three twinlinked lascannons (which are identical to railguns) and two heavy bolters, as opposed to 210 for three broadsides, each with one twinlinked railgun) that are also fast flying transport scouts with heavy bolters strapped to them.
If it came down to a shootout between a vendetta and a single squad of two broadsides, it would probably come down to whichever player got the first turn, unless there was cover high enough for the vendetta could get behind, in which case
it would almost certainly be the vendetta after an hour of furious calculations, I am a little less sure that it's so clear cut, but the vendetta still has the advantage. It has a 97% chance to kill one of the broadsides, a 66% chance to kill both, and a 24% chance to kill a theoretical third broadside two. With it's lascannons, that is. There's no way in hell I'm doing what I just did for the heavy bolters too (which is, computing the likelihood of a certain number of shots hitting, and then calculating for each of those the chance of each possible number of shots wounding. Now, I'm tired, and sick of number crunching, so this last step is probably wrong: I took the percentages for each number of wounds based on hit, and then combined them in the manner A+(1-A)*B=C and C+(1-C)*D=E, where C (or E) is the final percentage, depending on whether there were two or three numbers to combine. I have the feeling I'm wrong in making the assumption, and I believe I know the correct method, but quite frankly I don't give a shit right now, and it's close enough to give a decent idea of the odds). So yeah...
Regardless of the accuracy of the numbers, the broadsides would still be in a proportionally worse position, dying on a 2+ wound roll, having only two shots between them, inflicting a penetrating shot on a 3+ instead of 2+, and having neither guarantee a kill, or even damage. Then there's the heavy bolters to contend with, if the broadsides are within 36". Although wouldn't help too much against broadsides, they're still another factor tipping the scales in favor of the vendetta.
Against anything
other than broadsides, the vendetta is certainly more lethal, and carries much more utility, being a fast transport scout skimmer than can deepstrike and carries both anti-infantry and anti-armor weapons, all in a 12/12/10 shell.