Be careful with Vendettas; a good Tau player can right royally fuck you over if he knows Vendettas are coming (and since Vendettas are horribly OP every IG player fields them, so invariably he does), a proper deployment will prevent your Vendettas getting into range before he gets off at least one salvo with his Broadsides, and railguns will pop Vendettas like balloons.
A railgun will pop a leman russ too. Or a chimera. Or a monolith. If the map has enough cover, it would be trivial to move a vendetta or two into position and pop the broadsides. Not to mention that lascannons have a range of 48", which is the short length of the board. It's doubtful that they'd end up out of range unless the broadsides were hiding in a corner.
A heavy bolter fires three shots per attack, if it's a vet squad firing it'll hit on a 3+, meaning 2/3 of those three shots hit, for two hits per attack, on average. Autocannons fire two shots, 2/3 will hit, leaving you at 4/3 hits per attack. Heavy bolters are S5, so against T5 they'd have a 1/2 chance of inflicting a wound. Autocannons are S7, and so have a 5/6 chance of inflicting a wound against T5. The bolters will thus inflict 2/2 (so one) wound against a T5 per attack, and autocannons will inflict 10/9 wounds per attack. I use T5 because it's the turning point where autocannons become effective, they're also of more use against vehicles. Against T4, which is the toughest the tau have short of lol krootox, the bolters will inflict a wound 2/3 of the time, and autocannons 5/6 of the time, for 4/3 wounds per attack for the heavy bolters, and 10/9 for the autocannons, leaving the bolters slightly more effective. Both are AP4, and thus the saving throw is irrelevant to these calculations, both will perform the same against a given save. Even 10 armor is equivalent to T6 (well, marginally better...), so the autocannons is always the choice weapon for engaging vehicles, at least compared to heavy bolters.
Aqizzar: vendettas, not valkyries. Vendettas are worth a damn, and will slaughter the tau's heaviest hitting units, valkyries are not, and will be nothing more than annoying little sacks of shit buzzing around. And if you're not deploying them behind BLoS terrain on the first turn, you deserve to lose.
I think most of us are playing it on vassal, though I haven't seen many on recently.